NOCOMMIT: PoC: vectorize just expm1

[swolchok]

Here is how we can vectorize expm1 given the prior PRs in this stack. It is NOCOMMIT because we actually should implement vectorization for all the unary_ufunc_* ops.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> NOCOMMIT: PoC: vectorize just expm1 #9586
• relax tolerances for all unary float ops #9585
• Add vectorization in elementwise_util #9432
• BroadcastIndexesRange: leading 1s don't require true broadcasting #9431
• RFC: Specialize for non-mixed-dtype in elementwise_util #9388
• Add SupportedTensorDtypes::{BOOL,REALH} #9584
• Refactor elementwise_util: create variants with out_dtypes in template argument list #9387
• elementwise_util: don't cast the result of compute_fun back to the common type #9385
• Make PyTorch headers available in optimized_portable_kernels, define ET_USE_PYTORCH_HEADERS #9384

[pytorch-bot]

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