Fix use-after-free when a custom Metal kernel is called with different dtypes in one graph

[discobot]

Proposed changes

Fixes #3347.

Calling the same mx.fast.metal_kernel with different input dtypes inside one lazy graph regenerates different source under the same kernel name, and the cache eviction (clear_library) releases a pipeline state that an uncommitted command buffer still references (mechanism detailed in #3347). This implements the approach @zcbenz suggested on #3434: put the input dtypes in the generated kernel name, so a given name always maps to exactly one source and dtype variants coexist in the cache instead of evicting each other.

Dtypes turn out not to be the only per-call fact write_signature bakes into the source: the address space choice (constant for inputs with size() < 8) and pass-by-reference for 0-dim inputs vary per call too. The generated name now encodes all of it — each input's dtype with a c/s suffix for those two cases, plus the output dtypes — and max_constant_array_size is hoisted to file scope so the naming and write_signature can't drift. The clear_library path is kept for genuinely different user sources reusing one name (covered by the existing test_custom_kernel_caching).

Added a regression test (test_custom_kernel_mixed_dtypes) that composes float16 and float32 invocations of one kernel in a single graph; with METAL_DEVICE_WRAPPER_TYPE=1 as CI sets, the unfixed code aborts on it with the command buffer references deallocated object assertion. Also updated the generated-name example in the custom Metal kernels docs (names now look like custom_kernel_myexp_float_float16_float16).

With the fix, test_fast.py (24 tests) and test_export_import.py (16 tests) pass under Metal API validation.

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