[dynamo] Explicitly fall back to eager with GraphModule with no outpu…

…t for onnx&tvm backends (#99805)

Fixes #99437

Pull Request resolved: #99805
Approved by: https://github.com/jansel

torch/_dynamo/backends/onnxrt.py

@@ -60,6 +60,9 @@ def onnxrt(gm, example_inputs, *, filename=None, provider=None):
 
     device_type = device_from_inputs(example_inputs).type
     example_outputs = gm(*example_inputs)
+    if len(example_outputs) == 0:
+        log.warning("Explicitly fall back to eager due to zero output")
+        return gm.forward
     output_spec = [
         (o.shape, o.dtype, o.layout, o.device, o.requires_grad) for o in example_outputs
     ]

torch/_dynamo/backends/tvm.py

@@ -22,6 +22,10 @@ def tvm(gm, example_inputs, *, scheduler=None, trials=20000):
     jit_mod = torch.jit.trace(gm, example_inputs)
     device = device_from_inputs(example_inputs)
     shape_list = [(f"inp_{idx}", i.shape) for idx, i in enumerate(example_inputs)]
+    example_outputs = gm(*example_inputs)
+    if len(example_outputs) == 0:
+        log.warning("Explicitly fall back to eager due to zero output")
+        return gm.forward
     mod, params = relay.frontend.from_pytorch(jit_mod, shape_list)
     if device.type == "cuda":
         dev = tvm.cuda(device.index)