[inductor] Use full bit widths for bf16/fp16 vectorization

[CaoE]

Stack from ghstack (oldest at bottom):

• -> [inductor] Use full bit widths for bf16/fp16 vectorization #126502
• add specilizations for VecMaskLoad and VecMaskCast #126501
• add VectorizedN support for welford_vec_reduce_all and add float<->bf16/fp16 conversion specialization for VectorizedN #126500

cc @voznesenskym @penguinwu @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @wenzhe-nrv @jiayisunx @peterbell10 @ipiszy @yf225 @chenyang78 @kadeng @muchulee8 @ColinPeppler @amjames @desertfire @chauhang

[pytorch-bot]

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✅ You can merge normally! (9 Unrelated Failures)

As of commit 8afefb6 with merge base ab07867 ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• periodic / linux-focal-rocm6.1-py3.8 / test (distributed, 2, 2, linux.rocm.gpu) (gh) (disabled by #125504 but the issue was closed recently and a rebase is needed to make it pass)
  distributed/_tensor/test_attention.py::RingAttentionTest::test_ring_attention_compile_attention_fn0
• periodic / linux-vulkan-focal-py3.11-clang10 / test (default, 1, 1, linux.2xlarge) (gh) (similar failure)
  Process completed with exit code 1.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (aot_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird
• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (dynamic_aot_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird
• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (dynamo_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird
• periodic / linux-focal-cuda11.8-py3.9-gcc9 / test (multigpu, 1, 1, linux.g5.12xlarge.nvidia.gpu) (gh) (trunk failure)
  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!! KeyboardInterrupt !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

UNSTABLE - The following jobs failed but were likely due to flakiness present on trunk and has been marked as unstable:

• inductor / cuda12.1-py3.10-gcc9-sm86 / test (aot_inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird
• inductor / cuda12.1-py3.10-gcc9-sm86 / test (dynamic_inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird
• inductor / cuda12.1-py3.10-gcc9-sm86 / test (inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird

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