[Kernel][Hardware][Amd]Custom paged attention kernel for rocm

[charlifu]

This PR adds custom paged attention kernel support for rocm.

• add a separate folder for rocm kernel, which enables more specialized kernels for rocm in the future.
• add custom paged attention for rocm
• enable rocm custom paged attention kernel for specific size and dtype.
• add unit test

Todo:

• add fp8 kv cache support for rocm custom paged attention.
• add navi support for rocm custom paged attention.

Performance improvement (llama-2-70b-chat-hf):

num-prompt	input-len	output-len	w/o	w/
8	512	256	360.15 tokens/s	419.62 tokens/s
8	512	512	271.54 tokens/s	293.50 tokens/s
16	512	256	616.33 tokens/s	770.72 tokens/s
16	512	512	414.81 tokens/s	560.03 tokens/s
32	512	256	913.00 tokens/s	1263.37 tokens/s
32	512	512	626.82 tokens/s	969.73 tokens/s

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[WoosukKwon]

@charlifu Thanks for upstreaming the kernel and also adding the test!

It looks good to me overall, but I have a few suggestions:

1. Move use_rocm_paged_attention in selected.py to rocm_flash_attn.py.
2. Cache rocm_paged_attention_available in use_rocm_paged_attention to minimize its overheads.
3. In rocm_flash_attn.py, use rocm_paged_attention_available to decide between PagedAttention.forward_decode and ops.paged_attention_rocm. This way, we will be able to keep paged_attn.py untouched by this PR.

[WoosukKwon]

Is this because other head sizes are not supported? Or, is it just for reducing the cost?

[charlifu]

Other sizes are not supported yet, we will add them in subsequent PR.

[WoosukKwon]

LGTM! Thanks for addressing my review!