Reapply "Add vectorized_math.h (#11204)", "Add optimized_portable_kernels test (#11205)", and "Add vectorization in elementwise_util (#9432)"

[DariusHolmgren]

Summary:
Stack was reverted due to internal CI failures. Reapplying as an exported internal diff so that we make sure to catch any more of those.

New fixes:

• straightforward op_sub build fixes
• s/EXPECT_EQ/EXPECT_FLOAT_EQ/ in vectorized_math_test
• define ET_USE_PYTORCH_HEADERS to detect whether exceptions are
  enabled, and use #if defined(...) && ... instead of #ifdef to check the macro so
  that we don't use PyTorch headers if exceptions are
  disabled. (otherwise, we might have problems with e.g. TORCH_CHECK)

Original summary for #11204:
Set of math functions that work on both scalars and at::vec::Vectorized,
to be used in #9432.

Original summary for #11205:
Make sure we test the optimized versions of portable kernels even if
they are shadowed by optimized implementations. Intended to support
#9432.

Original summary for #9432:

This is a first cut at #9241 . In this PR I've vectorized a small
initial set of ops: atan2, clamp, fmod_Scalar, maximum, minimum, mul,
pow, and sigmoid. In addition, the following ops should have gotten
vectorized automatically because they already used generic lambdas: add,
div, rsub, sub. I've left covering ops that use the unary_ufunc_*
utilities in
pattern.h
for a follow-up push, because pattern.h and elementwise_util need some
work before we can migrate pattern.h's utilities to be backed by
elementwise_util.

This PR adds an interesting testing problem: in theory, all operators
might need test cases long enough to tickle vectorization, because we
might accidentally vectorize ops unexpectedly and break their lambdas
due to anticipated differences in semantics. I address this issue by
using Vectorized for the scalar prologue/epilogue in debug mode (we run
tests in both debug and release) so that we can detect broken lambdas. I
additionally intentionally introduced a bug in the vectorized path in
elementwise_util and manually verified that we saw test failures for
each vectorized op called out above.

Differential Revision:
D76467389

---

fix ET_USE_PYTORCH_HEADERS

[pytorch-bot]

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❌ 2 New Failures

As of commit 735f214 with merge base 56392aa ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  RuntimeError: Command docker exec -t fc1ebce4ad51b3a83161becab4e3867f6d347078fc53ecc35afef84bd4665b52 /exec failed with exit code 127
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D76467389

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