[feature] Add clip_grad_norm for hybrid_parallel_plugin

colossalai/amp/naive_amp/mixed_precision_optimizer.py

@@ -1,7 +1,7 @@
-from typing import Dict, List
+from typing import Dict, List, Tuple
 
 import torch
-from torch import Tensor
+from torch import Tensor, inf
 from torch.nn import Module, Parameter
 from torch.optim import Optimizer
 
@@ -68,8 +68,6 @@ def __init__(
             self.mixed_precision = BF16MixedPrecisionMixin()
         else:
             raise ValueError(f"Unsupported precision: {precision}")
-        if max_norm > 0.0:
-            raise NotImplementedError("max_norm is not supported yet.")
         self.max_norm = max_norm
         self.working_to_master_map: Dict[Parameter, Tensor] = {}
         self.master_to_working_map: Dict[Tensor, Parameter] = {}
@@ -102,32 +100,65 @@ def zero_grad(self, *args, **kwargs):
         return super().zero_grad(*args, **kwargs)
 
     def _unscale_and_clip_grads(self, total_norm: float) -> None:
+        """
+        Unscale and clip gradients before performing the optimization step.
+
+        Args:
+            total_norm (float): The computed total gradient norm.
+
+        Returns:
+            None
+        """
         div_scale = 1.0
+
+        # If mixed-precision training is used, get the gradient division scale from the mixed-precision handler.
         if self.mixed_precision is not None:
             div_scale = self.mixed_precision.get_grad_div_scale()
 
         if self.max_norm > 0.0:
-            # norm is in fact norm*scale
+            # Calculate the scaling factor for gradient clipping
+            # The gradient norm is scaled by 'div_scale' and then clipped to 'max_norm'
             clip = ((total_norm / div_scale) + 1e-6) / self.max_norm
+
+            # If the clip factor exceeds 1, adjust 'div_scale' accordingly to ensure clipping
             if clip > 1:
                 div_scale = clip * div_scale
 
+        # Apply the scaling factor to gradients
         for group in self.param_groups:
             for p in group["params"]:
                 if p.grad is None:
                     continue
                 p.grad.data.mul_(1.0 / div_scale)
 
-    def _compute_grad_norm(self) -> float:
-        if self.max_norm <= 0.0:
-            return 0.0
-        grads = [p.grad for group in self.param_groups for p in group["params"] if p.grad is not None]
-        if len(grads) == 0:
+    def _compute_grad_norm(self, param_gradient_pairs: List[Tuple[Tensor]], norm_type: int = 2) -> int:
+        r"""
+        Compute and return the gradient norm for gradient clipping.
+
+        Args:
+            param_gradient_pairs (List[Tuple[Tensor]]): List of (parameter, gradient) pairs; gradients are used for norm calculation.
+            norm_type (int, optional): Type of the norm used (e.g., 2 for L2 norm). Defaults to 2.
+
+        Returns:
+            float: The total norm of the given gradients.
+        """
+
+        if len(param_gradient_pairs) == 0:
             return 0.0
-        device = grads[0].device
-        # TODO(ver217): support tp
-        total_norm = torch.norm(torch.stack([torch.norm(g.detach(), 2).to(device) for g in grads]), 2)
-        return total_norm.item()
+
+        # gradients used for norm calculation.
+        gradients = [grad for param, grad in param_gradient_pairs]
+
+        if norm_type == inf:
+            total_norm = max(grad.data.abs().max() for grad in gradients)
+
+        else:
+            total_norm_exponentiated = 0.0
+            for grad in gradients:
+                total_norm_exponentiated += grad.data.double().norm(norm_type) ** norm_type
+            total_norm = total_norm_exponentiated ** (1.0 / norm_type)
+
+        return total_norm
 
     def step(self, *args, **kwargs):
         if self.mixed_precision.should_skip_step():
@@ -142,8 +173,22 @@ def step(self, *args, **kwargs):
                 if working_param.grad is not None:
                     p.grad = working_param.grad.data.float()
                     working_param.grad = None
-        total_norm = self._compute_grad_norm()
+
+        # gradient unscale and clip.
+        if self.max_norm <= 0:
+            # no need to compute gradient norm.
+            total_norm = 0.0
+        else:
+            # compute the total norm.
+            param_gradient_pairs = [
+                (self.master_to_working_map[p], p.grad)
+                for group in self.param_groups
+                for p in group["params"]
+                if p.grad is not None
+            ]
+            total_norm = self._compute_grad_norm(param_gradient_pairs)
         self._unscale_and_clip_grads(total_norm)
+
         self.optim.step(*args, **kwargs)
         # update working params
         for group in self.optim.param_groups: