Fix Python ctypes signatures

[fallintoplace]

Summary

This fixes Python ctypes signatures for generated host functions in the CUTLASS Python interface.

The fresh compile path was assigning func.argtype, which is just a normal Python attribute and does not configure ctypes argument marshaling. The cache load path also restored _get_params functions without setting argument types. This change installs argtypes in both paths.

For grouped GEMM host precompute, the third generated C++ argument is size_t workspace_bytes, so the Python signature now uses ctypes.c_size_t instead of ctypes.c_longlong.

Validation

• PYTHONPATH=python python -m unittest test/python/cutlass/backend/test_ctypes_signature.py
• PYTHONPATH=python python -m py_compile python/cutlass_cppgen/backend/compiler.py python/cutlass_cppgen/backend/gemm_operation.py test/python/cutlass/backend/test_ctypes_signature.py