Improve pipeline partitioning

[hmellor]

As discussed in https://vllm-dev.slack.com/archives/C08AD2B5HH8/p1740503158346139

If the number of layers is not divisible by the number of partitions, the remaining layers are evenly distributed across all but the last partition. The last partition is excluded because it often contains an additional norm layer and we are attempting to balance compute.

If pp_size > 2 and the number of remaining layers is 0 < x <= pp_size - 2 then the remaining layers are evenly distributed across the middle partitions. The first and last partitions are excluded because they contain the input and output embeddings respectively and we are attempting to reduce maximum memory consumption across partitions.

The following screenshot shows how the remaining layers are distributed for all possible numbers of remaining layers:

Fixes #13045

[github-actions]

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[comaniac]

Overall LGTM. Should we have a unit test?

[mgoin]

Please add a test to tests/distributed/test_pipeline_partition.py

[hmellor]

Unit tests added

[youkaichao]

LGTM. the plugin test hangs due to the bug from main, and is fixed by #13832 .