sequence parallel for uneven heads (#6392)
In sequence_parallel (Ulysses), the sequence parallel size is
constrained by the requirement to be divisible by the number of heads,
which prevents some models/workloads from setting a specific sequence
parallel size. This PR implements uneven all-to-all heads splitting.
- both support batch first (b,s,...) and seq_len first(s,b..) layout.
- Added unit tests with numerical checks. Locally also tested with **7
heads with sp=4** and **20 heads with sp=8**, and it passed.
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Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
Co-authored-by: Ma, Guokai <guokai.ma@gmail.com>
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
