Add missing defaults to tri, tril, triu, gather_mm signatures

[rishabhsai]

The signatures for tri, tril, triu, and gather_mm omit defaults that their Python bindings already provide. The generated stubs therefore mark these arguments as required, causing Pyright to reject valid calls such as mx.tri(3) and mx.gather_mm(x, x).

This updates the four nb::sig strings in python/src/ops.cpp to match the existing bindings and docstrings:

• tri: declare m: int | None = None and k: int = 0.
• tril and triu: declare k: int = 0.
• gather_mm: declare both index arguments as array | None = None.

There is no runtime behavior change.

Validation used a CPU-only source build. The regenerated stubs contain the corrected signatures, and Pyright reports zero errors on six example calls that previously produced six missing-argument errors. Those calls execute successfully both before and after the change. The existing tri, tril, and triu tests pass, as does the clang-format check.