Description

DeepCompile retrieves PyTorch's compiled backward hooks directly from the class namespace. On Python 3.9, this returns a staticmethod descriptor that cannot be
called directly, resulting in:

TypeError: 'staticmethod' object is not callable

This change unwraps the underlying function before DeepSpeed invokes it.

The fix covers both supported implementations:

• PyTorch 2.6: _backward_prologue
• PyTorch 2.7+: _backward_impl

A regression test emulates Python 3.9's non-callable staticmethod behavior so the failure remains covered when CI runs on newer Python versions.

Testing

• pytest -q tests/unit/compile/test_backend.py tests/unit/compile/test_zero3_grad_dtype.py
• pre-commit run --files deepspeed/compile/patch_compiled_func.py tests/unit/compile/test_backend.py

Fixes #7433