[BugFix] Fix vllm_flash_attn install issues

[LucasWilkinson]

This PR is a collection of fixes for vllm_flash_attn install issues, unfortunately the vllm_flash_attn install is fairly hacky/sensitive/complex right now. This will hopefully be fixed in the future if we move to a separate kernel library. Apologies for missing #17159 .

1. Fix Python packaging edge cases #17159 introduced an __init__.py into the vllm_flash_attn, during the install process we copy over the __init__.py file from the vllm_flash_attn repo https://github.com/vllm-project/flash-attention. This creates a bad diff since vllm/vllm_flash_attn/__init__.py was now being tracked by git. PRs [Chore] added stubs for vllm_flash_attn during development mode #17228 and [Chore] ignore override default __init__.py when building from source #17260 attempted to fix this by not coping the __init__.py from vllm_flash_attn repo. This is error prone because it requires us to keep the stub files in-sync between the repos. I am assuming Fix Python packaging edge cases #17159 introduced an __init__.py to make sure vllm/vllm_flash_attn/fa_utils.py got packaged correctly since this is the only python file that would exist in that folder if __init__.py from the vllm_flash_attn repo was not copied over. As an alternative in this PR I moved that file to vllm/attention/utils/fa_utils.py, this way it always gets packaged regardless if vllm_flash_attn is available, meaning we can remove the __init__.py from the folder and rely on the one that gets copied from vllm_flash_attn repo (if its not present we dont care about packaging this folder anymore). (NOTE this PR reverts [Chore] added stubs for vllm_flash_attn during development mode #17228)

2. [Perf]Optimize rotary_emb implementation to use Triton operator for improved inference performance #16457 and Add rotary triton operator to vllm_flash_attn flash-attention#64 introduced new python files to vllm_flash_attn but these were not getting properly picked up by setup.py, this PR now recursively globs for .py files in vllm_flash_attn making sure no files are inadvertently missed (FIX [Bug]: nightly version: ModuleNotFoundError: No module named 'vllm.vllm_flash_attn.layers' #17263), Note this PR supersedes: [Bugfix] Fix vllm_flash_attn rotary import  #17247 which achieves the same result albeit in more lines of code (shout-out to @jeejeelee and @aarnphm for highlighting the issue).

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[aarnphm]

This is clean. LGTM. I will close the other PR

[houseroad]

@zhewenl, we will need to update the build script again. Probably it's me, since I will be the oncall.

[tlrmchlsmth]

Thanks for fixing this