[dynamo] Add specialized variable tracker for sys.modules (#110990)
`sys.modules` is currently treated as a constant dictionary and any reference to
it will result in guards on the full contents of `sys.modules`. This instead
adds a specialized variable tracker which tries to guard only on the modules
referenced by the code. e.g.
```
sys.modules["operator"].add(x, x)
```
will generate the guard
```
___dict_contains('operator', G['sys'].modules)
```
It does this with special support for `__contains__` `__getitem__` and `.get`
which are probably the most commonly used with `sys.modules`. For anything else
we just fall back to building the dict tracker as normal.
While accessing `sys.modules` may seem unusual, it actually comes up when
inlining the `warnings.catch_warnings` context manager which internally accesses
`sys.modules["warnings"]`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110990
Approved by: https://github.com/ezyang
