Add typing annotations to guard and source

Summary:
X-link: pytorch/executorch#12986

As part of better engineering week, we would like to improve out type support to improve dev experience in dynamo

This PR adds strict typing support to a critical set of files for dynamo, `source.py` and the base `_guards.py`

Running
```
mypy torch/_dynamo/source.py torch/_guards.py --linecount-report /tmp/coverage_log
```

| -------- | Lines Unannotated | Lines Total | % lines covered | Funcs Unannotated | Funcs Total | % funcs covered |
| -------- | ------- | -------- | ------- | ------- | ------- | ------- |
| Main  |  1227 | 2208 | 55.57% | 207 | 362 | 57.18% |
| This PR | 2217 | 2217 | 100.00% | 362 | 362 | 100.00% |
| Delta    | +990 | +9 | +44.43% | +155 | 0 | +42.82% |

cc jgong5 mingfeima XiaobingSuper sanchitintel ashokei jingxu10 jerryzh168 voznesenskym penguinwu EikanWang Guobing-Chen zhuhaozhe blzheng wenzhe-nrv jiayisunx ipiszy chenyang78 kadeng muchulee8 amjames chauhang aakhundov coconutruben

X-link: pytorch/pytorch#158397

Reviewed By: JacobSzwejbka, yangw-dev

Differential Revision: D79199389

Pulled By: Lucaskabela

fbshipit-source-id: 10b5d9d6ddb4b0797ea795ac8a1f1eaaa512b2ae

Author: Lucaskabela

userbenchmark/dynamo/dynamobench/_dynamo/utils.py

@@ -2644,7 +2644,9 @@ def set_example_value(node, example_value):
     # this to accurately reflect what the state of the value was at the time
     # the program was traced).
     node.meta["example_value"] = example_value
-    shape_env = TracingContext.get().fake_mode.shape_env
+    fake_mode = TracingContext.get().fake_mode
+    assert fake_mode is not None
+    shape_env = fake_mode.shape_env
     if (
         symbol_to_path
         := torch.fx.experimental.symbolic_shapes.compute_unbacked_bindings(
@@ -4765,7 +4767,7 @@ def record_pregraph_bytecode_exit(cm: AbstractContextManager[None]) -> None:
 
 # Returns a set of code objects present traced in the current TracingContext, or None
 # if there is no current TracingContext.
-def get_traced_code() -> list[CodeType]:
+def get_traced_code() -> Optional[list[CodeType]]:
     from torch._guards import TracingContext
 
     return TracingContext.get_traced_code()