[FSDP] Slightly refactor fx symbolic tracer (#89917)
I made a pass over Linjian's `_symbolic_trace.py` and tidied it up a bit. Aside from simple stylistic changes, this PR makes the following changes:
- Save `visited_params: Set[nn.Parameter]` to avoid linear overhead to check a parameter already being visited when appending to the parameter execution order list (`param_forward_order`)
- Move the tracer patching logic to a class `_ExecOrderTracer` to have a reference to `self.exec_info` without having a fragmented 2-step initialization (like the old `_init_execution_info(root_module)` plus `_patch_tracer(tracer, root_module, execution_info)`)
- Define `_ParamUsageInfo` to formalize the `Tuple[nn.Module, List[str, nn.Parameter]]` elements being mapped to in the execution info `dict`, and clarify the documentation regarding what this represents
- Change the unit test to use `TestCase`, not `FSDPTest`, to avoid initializing a process group
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89917
Approved by: https://github.com/zhaojuanmao, https://github.com/fegin
