[FX] Specifies a default value when possible for placeholders created from concrete_args (#59569)
Summary:
```python
class Foo(torch.nn.Module):
def __init__(self):
super().__init__()
def forward(self, a=None, b=None):
res = a
if b is not None:
res = res + b
return res
concrete_args = {'b': torch.tensor(5)}
traced = fx.symbolic_trace(Foo(), concrete_args=concrete_args)
```
Gives the following error:
```
File "<eval_with_key_9>", line 2
def forward(self, a = None, b_1):
^
SyntaxError: non-default argument follows default argument
```
Since https://github.com/pytorch/pytorch/issues/55888, placeholders are also created for concrete arguments. But these placeholders do not have default values even when it was provided for the argument in question, causing the error above.
To solve this, I add a default value when it is available during placeholder creation for concrete arguments.
I also tried to set the default value to the value specified in concrete_args (since it many cases it will actually use this value anyway), but ran into an error because the default value is never defined:
```
def forward(self, a = None, b_1 = _tensor_constant0):
_tensor_constant0 = self._tensor_constant0
_tensor_constant1 = self._tensor_constant1
add = a + _tensor_constant1; a = _tensor_constant1 = None
NameError: name '_tensor_constant0' is not defined
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59569
Reviewed By: albanD
Differential Revision: D31385607
Pulled By: Chillee
fbshipit-source-id: 44a8ce28b5eabdb9b4c773e73a68ff0bb9c464cc
