[Bug]: Quantization In MambaMixer2 Not Supported when Tensor Parallel is enabled

[fabianlim]

Your current environment

The output of `python collect_env.py`

Environment is not relevant for this issue.

🐛 Describe the bug

The current implementation for TP for Mamba2 is complicated for the in_proj, because the gate, projection, state space, heads, are all fused into this one layer. And furthermore, we also need to consider different possibilities if the number of groups divide the number of heads or not, see #13660.

For now the implementation of TP is simplified:

• limited to the case of num_groups == 1 if num_groups does not divide num_heads #13660.
• will does not support TP > 1 if the mamba2 mixer is quantised, see #14617

However for large models, it may be useful to support TP > 1 with quant layers, even in some special cases of num_heads and num_groups. cc: @tlrmchlsmth

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