fix: use identity check for None comparison (PEP 8 E711)

[harshadkhetpal]

Summary

Replace == None with is None identity check in model_worker.py per PEP 8 (E711).

Python's is operator is the recommended way to compare with singletons like None. Using == can produce unexpected results if a class defines a custom __eq__ method.

Change:

# Before
if self.tokenizer.pad_token == None:

# After
if self.tokenizer.pad_token is None:

Testing

No behavior change — identity check with is is functionally equivalent to == for None comparisons in standard usage.