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b8d35ba5 - dynamo: support next() on itertools.count inputs (#177876)

dynamo: support next() on itertools.count inputs (#177876) Summary: Fix #177196 1. Root cause Dynamo wrapped pre-existing `itertools.count()` objects as generic user-defined objects, so `next(counter)` fell through to the unsupported `__next__` path. The existing iterator side-effect replay also had no way to keep a live `count()` object synchronized after compiled execution. 2. Proposed fix Wrap pre-existing `itertools.count()` instances as `CountIteratorVariable`, guard them on their current iterator state, and replay consumed `next()` calls back onto the original iterator after the compiled region. Add regression tests for the external-counter fullgraph case and the new guard. 3. Why the proposed fix is the right long term fix This keeps `itertools.count()` on Dynamo's existing iterator path instead of treating it like an opaque user object. That gives Dynamo the same three things it already needs for mutable iterators: a dedicated variable type during tracing, guard-based cache invalidation when iterator state changes, and side-effect replay that preserves the original Python object. Drafted via Codex, published after manual review by bobrenjc93 X-link: https://github.com/pytorch/pytorch/pull/177876 Approved by: https://github.com/Lucaskabela Reviewed By: georgehong Differential Revision: D98188470 fbshipit-source-id: d470e75b36fea669c16ade5c7c43205ba4b44410