Fix explicit sharding for moe models

[Shuwen-Fang]

Description

This PR fixes explicit sharding for deepseek by specifying correct sharding for expert weights, and add tests with ds3-test for both the sparse and dense matmul path.

Tests

• Verified pytest tests/integration/smoke/train_smoke_test.py::Train -v passed

Tested the following configs to validate correctness with deepseek3-test:

ici_fsdp_parallelism=2 \
ici_expert_parallelism=8 \
use_ring_of_experts=false \

ici_fsdp_parallelism=2 \
ici_expert_parallelism=8 \
use_ring_of_experts=false \

ici_fsdp_parallelism=4 \
ici_tensor_parallelism=4 \

ici_fsdp_parallelism=16 \

ici_fsdp_parallelism=8\
ici_tensor_transpose_parallelism=2 \

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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