Adding uint4 dtype implementation

Summary:
We have a lot of interest for int4 dtypes, and we'd like to add the dtype out of PyTorch core.
This PR added some preliminary support for uint4 through tensor subclass and we'll continue to iterate on this

Test Plan:
python test/dtypes/test_int4.py

Reviewers:

Subscribers:

Tasks:

Tags:

ghstack-source-id: 812be7a47b59c8e78f2b8d1d74f8bb04f104091d
Pull Request resolved: #13

test/dtypes/test_int4.py

@@ -0,0 +1,322 @@
+import torch
+from torchao.dtypes.int4 import UInt4Tensor
+import unittest
+from unittest import TestCase, main
+from torch.ao.quantization.quantize_pt2e import prepare_pt2e, convert_pt2e
+from torch.ao.quantization.quantizer import QuantizationSpec, Quantizer
+
+from torch._export import capture_pre_autograd_graph
+from torch._export import dynamic_dim
+from torch.testing._internal.common_quantization import (
+    NodeSpec as ns,
+    QuantizationTestCase,
+)
+from torchao.quantization.utils import (
+    compute_error,
+)
+from torchao.quantization.quant_api import (
+    replace_with_custom_fn_if_matches_filter,
+)
+from torch.ao.quantization.observer import ObserverBase
+from torch import nn
+import copy
+
+def _dynamically_quantize_per_channel_int4(x, quant_min, quant_max, target_dtype):
+    # assumes symmetric quantization
+    # assumes axis == 0
+    # assumes dense memory format
+    # TODO(future): relax ^ as needed
+
+    # default setup for affine quantization of activations
+    eps = torch.finfo(torch.float32).eps
+
+    # get min and max
+    min_val, max_val = torch.aminmax(x, dim=1)
+
+    # calculate scale and zero point based on min and max
+    # reference: https://fburl.com/code/srbiybme
+    min_val_neg = torch.min(min_val, torch.zeros_like(min_val))
+    max_val_pos = torch.max(max_val, torch.zeros_like(max_val))
+    device = min_val_neg.device
+
+    # reference: https://fburl.com/code/4wll53rk
+    max_val_pos = torch.max(-min_val_neg, max_val_pos)
+    scale = max_val_pos / (float(quant_max - quant_min) / 2)
+    # ensure scale is the same dtype as the original tensor
+    scale = torch.clamp(scale, min=eps).to(x.dtype)
+    zero_point = torch.zeros(min_val_neg.size(), dtype=torch.int64, device=device)
+
+    # quantize based on qmin/qmax/scale/zp
+    # reference: torch/ao/quantization/fx/_decomposed.py?lines=63
+    x_div = x.transpose(0, 1) / scale
+    x_round = torch.round(x_div)
+    x_zp = x_round + zero_point
+    x_zp = x_zp.transpose(0, 1)
+    quant = torch.clamp(x_zp, quant_min, quant_max)
+    if target_dtype == "int4":
+        quant = UInt4Tensor.from_unpacked(quant.to(torch.uint8)).view(quant.size())
+    else:
+        quant = quant.to(target_dtype)
+
+    return quant, scale, zero_point
+
+class _WeightOnlyInt4QuantLinear(torch.nn.Linear):
+    def __init__(self, *args, **kwargs):
+        w_int4 = kwargs.pop("w_int4")
+        scales = kwargs.pop("scales")
+        super().__init__(*args, **kwargs)
+        self.w_int4 = w_int4
+        self.scales = scales
+
+    def forward(self, x):
+        # if len(x.shape)<=2:
+        #     y = torch.mm(x, self.w_int8.to(x.dtype)) * self.scales
+        # else: # turn x into 2d tensor, then undo it for y
+        x_view = x.view(-1, x.shape[-1])
+        y = torch.mm(x_view, self.w_int4.to(torch.uint8).to(x.dtype)) * self.scales
+        y = y.reshape(*x.shape[:-1], -1)
+        if self.bias is not None:
+            y += self.bias
+        return y
+
+    @classmethod
+    def from_float(cls, mod):
+        w_fp32 = mod.weight
+        w_int4, scales, _zp = _dynamically_quantize_per_channel_int4(
+            w_fp32, 0, 15, "int4"
+        )
+        # create the new module with a toy size to ensure initialization is fast
+        fake_in_features, fake_out_features = 8, 8
+        new_mod = cls(
+            fake_in_features,
+            fake_out_features,
+            bias=mod.bias is not None,
+            # w_int4=w_int4.t().contiguous(),
+            w_int4=torch.ops.aten.transpose_copy(w_int4, 0, 1),
+            scales=scales,
+        )
+        new_mod.in_features = mod.in_features
+        new_mod.out_features = mod.out_features
+        del new_mod.weight
+        new_mod.bias = mod.bias
+        device_to_use = next(mod.parameters()).device
+        new_mod.to(device_to_use)
+        return new_mod
+
+def _apply_weight_only_int4_quant(model):
+    replace_with_custom_fn_if_matches_filter(
+        model,
+        _WeightOnlyInt4QuantLinear.from_float,
+        lambda mod, fqn: isinstance(mod, torch.nn.Linear),
+    )
+
+from torch.library import Library, impl
+
+test_lib = Library("test_int4", "DEF")
+test_lib.define("quantize_per_tensor_int4(Tensor input, float scale, int zero_point) -> Tensor")
+
+@impl(test_lib, "quantize_per_tensor_int4", "CompositeExplicitAutograd")
+def quantize_per_tensor_int4(
+    input: torch.Tensor,
+    scale: float,
+    zero_point: int,
+) -> torch.Tensor:
+    inv_scale = 1.0 / scale
+    return torch.clamp(torch.round(input * inv_scale) + zero_point, 0, 15).to(torch.uint8).view(torch.bits8)
+
+test_lib.define("dequantize_per_tensor_int4(Tensor input, float scale, int zero_point) -> Tensor")
+@impl(test_lib, "dequantize_per_tensor_int4", "CompositeExplicitAutograd")
+def dequantize_per_tensor_int4(
+    input: torch.Tensor,
+    scale: float,
+    zero_point: int,
+) -> torch.Tensor:
+    return (input.view(torch.uint8).to(torch.float32) - zero_point) * scale
+
+
+class TestInt4(QuantizationTestCase):
+    def test_basic_tensor_ops(self):
+        x = UInt4Tensor(torch.tensor([
+            [0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF],
+            [0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF],
+            [0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF],
+        ], dtype=torch.uint8))
+        self.assertTrue(x.shape, (3, 8))
+        # making sure these works
+        x.to(torch.uint8)
+        expected = UInt4Tensor(torch.tensor([
+            [0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF],
+        ], dtype=torch.uint8))
+        self.assertTrue(x[0:1, :] == expected)
+        expected = UInt4Tensor(torch.tensor([
+            [0x23, 0x45],
+            [0x23, 0x45],
+            [0x23, 0x45],
+        ], dtype=torch.uint8))
+        self.assertTrue(x[:, 2:6] == expected)
+
+    def test_gpu_quant(self):
+        for x_shape in [[2, 4], [5, 5, 5, 4], [1, 4, 4]]:
+            x = torch.randn(*x_shape)
+            m = nn.Sequential(nn.Linear(4, 16))
+            y_ref = m(x)
+            _apply_weight_only_int4_quant(m)
+            y_wo = m(x)
+            # sqnr = compute_error(y_ref, y_wo)
+            opt = torch.compile(m, mode="max-autotune")
+            # make sure it runs
+            opt(x)
+
+    def test_aten_ir(self):
+        # class QuantizePerTensorUInt4(torch.autograd.Function):
+        #     @staticmethod
+        #     def forward(
+        #         ctx,
+        #         input: torch.Tensor,
+        #         scale: float,
+        #         zero_point: int,
+        #     ) -> torch.Tensor:
+        #         inv_scale = 1.0 / scale
+        #         return UInt4Tensor(torch.clamp(torch.round(input * inv_scale) + zero_point, 0, 15).to(torch.uint8))
+
+        # class DeQuantizePerTensorUInt4(torch.autograd.Function):
+        #     @staticmethod
+        #     def forward(
+        #         ctx,
+        #         input: torch.Tensor,
+        #         scale: float,
+        #         zero_point: int,
+        #     ) -> torch.Tensor:
+        #         return (input.to(torch.float32) - zero_point) * scale
+
+        class M(torch.nn.Module):
+            def forward(self, x, y):
+                return x + y
+
+        example_inputs = (torch.randn(1, 2, 3, 3), torch.randn(1, 2, 3, 3),)
+        m = M().eval()
+        m = capture_pre_autograd_graph(m, example_inputs)
+        for n in m.graph.nodes:
+            if n.target == torch.ops.aten.add.Tensor:
+                with m.graph.inserting_before(n):
+                    q = m.graph.call_function(torch.ops.test_int4.quantize_per_tensor_int4, (n.args[0], 1.0, 0), {})
+                    dq = m.graph.call_function(torch.ops.test_int4.dequantize_per_tensor_int4, (q, 1.0, 0), {})
+                    n.replace_input_with(n.args[0], dq)
+        m.recompile()
+
+    def test_pt2e_quant(self):
+        from torch.ao.quantization.quantizer.xnnpack_quantizer_utils import (
+            OP_TO_ANNOTATOR,
+            QuantizationConfig,
+        )
+        class int4_class():
+            pass
+
+        torch.int4 = int4_class()
+
+        class Int4Observer(ObserverBase):
+            def __init__(self, *args, **kwargs):
+                # just faking a dtype here
+                # TODO: make flow work with new dtypes
+                super().__init__(dtype=torch.int8)
+
+            def forward(self, x):
+                return x
+
+            def calculate_qparams(self, **kwargs):
+                pass
+
+            def convert(self, model: torch.fx.GraphModule, observer_node: Node):
+                with model.graph.inserting_before(observer_node):
+                    q_node = model.graph.call_function(
+                        torch.ops.test_int4.quantize_per_tensor_int4, (observer_node.args[0], 1.0, 0), {})
+                    dq_node = model.graph.call_function(
+                        torch.ops.test_int4.dequantize_per_tensor_int4, (q_node, 1.0, 0), {})
+                    observer_node.replace_all_uses_with(dq_node)
+                    model.graph.erase_node(observer_node)
+
+        class Int4WeightQuantizer(Quantizer):
+            def annotate(self, model: torch.fx.GraphModule) -> torch.fx.GraphModule:
+                int4_qspec = QuantizationSpec(
+                    dtype=torch.int4,
+                    quant_min=-2**3,
+                    quant_max=2**3 - 1,
+                    qscheme=torch.per_tensor_affine,
+                    is_dynamic=False,
+                    observer_or_fake_quant_ctr=Int4Observer,
+                )
+                int8_qspec = QuantizationSpec(
+                    dtype=torch.int8,
+                    quant_min=-128,
+                    quant_max=127,
+                    qscheme=torch.per_tensor_symmetric,
+                    is_dynamic=False,
+                    observer_or_fake_quant_ctr=observer.default_weight_observer,
+                )
+                quantization_config = QuantizationConfig(
+                    input_activation=int8_qspec,
+                    weight=int4_qspec,
+                    bias=None,
+                    output_activation=int8_qspec,
+                )
+                OP_TO_ANNOTATOR["conv"](model, quantization_config)
+
+            def validate(self, model: torch.fx.GraphModule) -> None:
+                pass
+
+        class M(torch.nn.Module):
+            def __init__(self):
+                super().__init__()
+                self.conv = torch.nn.Conv2d(3, 3, 3)
+
+            def forward(self, x):
+                return self.conv(x)
+
+        quantizer = Int4WeightQuantizer()
+        node_occurrence = {
+            # for weight
+            torch.ops.test_int4.quantize_per_tensor_int4: 1,
+            torch.ops.test_int4.dequantize_per_tensor_int4: 1,
+            # for activation
+            torch.ops.quantized_decomposed.quantize_per_tensor.default: 2,
+            torch.ops.quantized_decomposed.dequantize_per_tensor.default: 2,
+        }
+        node_list = [
+            torch.ops.quantized_decomposed.dequantize_per_tensor.default,
+            torch.ops.test_int4.dequantize_per_tensor_int4,
+            torch.ops.aten.conv2d.default,
+            torch.ops.quantized_decomposed.quantize_per_tensor.default,
+        ]
+        example_inputs = (torch.randn(1, 3, 3, 3),)
+
+        # _test_quantizer in PT2EQuantizationTestCase
+        # resetting dynamo cache
+        export_with_dynamic_shape = False
+        torch._dynamo.reset()
+        m_eager = M().eval()
+
+        # program capture
+        m = copy.deepcopy(m_eager)
+        m = capture_pre_autograd_graph(
+            m,
+            example_inputs,
+        )
+
+        m = prepare_pt2e(m, quantizer)
+        # Calibrate
+        m(*example_inputs)
+        m = convert_pt2e(m, fold_quantize=True)
+
+        pt2_quant_output = m(*example_inputs)
+        node_occurrence = {
+            ns.call_function(k): v for k, v in expected_node_occurrence.items()
+        }
+        if expected_node_list is None:
+            expected_node_list = []
+        node_list = [ns.call_function(n) for n in expected_node_list]
+        self.checkGraphModuleNodes(
+            m, expected_node_occurrence=node_occurrence, expected_node_list=node_list
+        )
+
+if __name__ == "__main__":
+    main()

torchao/dtypes/__init__.py

@@ -0,0 +1,5 @@
+from .int4 import UInt4Tensor
+
+__all__ = [
+    "UInt4Tensor"
+]