refactor: group mm_embeddings per sequence at worker output.

[wly-115]

Description

Move multimodal embedding grouping from the batch consumer into the worker
output. SampleOutput.mm_embeddings is now grouped by sequence
(vector<vector<Tensor>>, outer = sequence, inner = one tensor per encoded
item in that sequence: images for mm_embed, the full hidden for the
full-embedding task), and each sequence's embeddings are carried on its own
RawSampleOutput. Serialization (shared memory and RPC) now maps the outer
sequence to outputs[i] instead of re-slicing a batch-level tensor list in
the consumer.

Related Issues

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactor
• Documentation
• Test
• Build or CI

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