[DeepSeek] Move seqlen from model config to setup_symm_mem

[kwen2501]

Just a UI change. The seqlen is mainly used in setting up symm mem. That's why we move it out of model config.

[lessw2020]

lgtm!
One note - the changes in symm_mem_recipes are assumed but not present here in this PR as a change nor in main? (I had to hand add those to get things to run).

verified changes as well:

Creating model stage 0 of 2
Creating model stage 1 of 2
Creating model stage 0 of 2
Creating model stage 1 of 2
EP rank [0]: Created Symmetric Memory for MoE
EP rank [1]: Created Symmetric Memory for MoE
EP rank [1]: Created Symmetric Memory for MoE
EP rank [0]: Created Symmetric Memory for MoE
NCCL version 2.25.1+cuda12.2
NCCL version 2.25.1+cuda12.2
NCCL version 2.25.1+cuda12.2
<｜begin▁of▁sentence｜>You are a helpful assistant.

User: What is 2+2?

Assistant: 2 + 2 equals 4.<｜end▁of▁sentence｜>

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