[ONNX] Inline prim::PythonOp for Autograd Function Export (#74765)
Add flag (inline_autograd) to enable inline export of model consisting of autograd functions. Currently, this flag should only be used in TrainingMode.EVAL and not for training.
An example:
If a model containing ``autograd.Function`` is as follows
```
class AutogradFunc(torch.autograd.Function):
@staticmethod
def forward(ctx, i):
result = i.exp()
result = result.log()
ctx.save_for_backward(result)
return result
```
Then the model is exported as
```
graph(%0 : Float):
%1 : Float = ^AutogradFunc(%0)
return (%1)
```
If inline_autograd is set to True, this will be exported as
```
graph(%0 : Float):
%1 : Float = onnx::Exp(%0)
%2 : Float = onnx::Log(%1)
return (%2)
```
If one of the ops within the autograd module is not supported, that particular node is exported as is mirroring ONNX_FALLTHROUGH mode
Fixes: #61813
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74765
Approved by: https://github.com/BowenBao, https://github.com/malfet
