Optimize MOE GEMV kernel for BS > 1. (#20905)

* Optimize MOE GEMV kernel for BS > 1.

The previous MOE kernel for BS > 1 had too many thread blocks (nrows_x, nchannels_dst, ncols_dst), with very little work per block. block of (32, 4) was doing inner dot product for a single row.

New mul_mat_vec_q_moe kernel is dedicated for MoE multi-token kernel with grid (ceil(nrows_x/rpb), nchannels_dst), block (warp_size, ncols_dst). Each warp handles two rows independently with warp-level reduction only (no shared memory sync).

This change doesn't increase any compilation time as a single template instance is needed per type. This also simplifies the original GEMV kernel and gets rid of `is_multi_token_id` specialization.

* Remove em-dashes

* Cherry-pick changes from @am17an PR https://github.com/ggml-org/llama.cpp/pull/20885 to enable small_k optimization only for cases where it benefits

Increase max batch size for MMVQ kernels for MUL_MAT_ID to 8

* Make the max batch size for MOE GEMV kernel configurable based on GPU arch and datatype

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Co-authored-by: Aman Gupta <[email protected]>