add VectorizedN support for welford_vec_reduce_all and add float<->bf16/fp16 conversion specialization for VectorizedN

[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 @jgong5 @mingfeima @XiaobingSuper @sanchitintel @ashokei @jingxu10 @voznesenskym @penguinwu @EikanWang @Guobing-Chen @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! (7 Unrelated Failures)

As of commit 3b23540 with merge base ab07867 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / macos-13-py3-arm64 / test (default, 3, 3, macos-m1-stable) (gh) (similar failure)
  Process completed with exit code 1.

BROKEN TRUNK - The following jobs failed but was 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

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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[peterbell10]

Suggested change

	auto result0 = welford_vec_reduce_all(Welford0);
	auto result1 = welford_vec_reduce_all(Welford1);
	return welford_combine(result0, result1);
	return welford_vec_reduce_all(welford_combine(Welford0, Welford1));