cuda: GET_ROWS quants (#25962)

cuda: GET_ROWS quants (#25962)

* cuda: add k-quant support to GET_ROWS

Device-side embedding lookups require GET_ROWS to handle the k-quants
used by common GGUF recipes (Q4_K_M stores token_embd as q6_K). Without
it the backend rejects the op and the scheduler falls back to the host,
copying the full embedding matrix back on every token in single-device
graphs.

Factor the super-block dequantizers out of the dequantize_block kernels
in convert.cu into shared device functions in dequantize.cuh and reuse
them from a new k_get_rows_kq kernel : one thread block dequantizes one
(dst row, super-block) pair with the existing thread layouts, 32 threads
for q4_K and 64 for the other k-quants.

Covers q2_K to q6_K in get_rows_cuda and supports_op. i-quants are left
as a TODO.

* cuda: add i-quant support to GET_ROWS

Extends the shared super-block dequantizers to the nine i-quants and
reuses them from k_get_rows_kq with the 32-thread layout of the matching
convert.cu kernels. supports_op gates the k-quant and i-quant path on
ne0 being a multiple of QK_K, which iq4_nl does not guarantee on its
own (QK4_NL sub-blocks). mxfp4 is left as a TODO.

* cuda: add mxfp4 support to GET_ROWS

Moves the mxfp4 dequantizer into the shared super-block helpers and
reuses it from k_get_rows_kq with the 32-thread layout of the matching
convert.cu kernel. mxfp4 joins the ne0 % QK_K gate in supports_op since
its 32-value sub-blocks do not guarantee QK_K-aligned rows on their own.
This closes GET_ROWS type coverage on CUDA: every quantized GGML type
now takes the direct device path.

* cuda: gate the GET_ROWS row size only for 32-value sub-block types

Address review from @pwilkin: the i-quant commit replaced the return
shared by the whole supported type cascade, so f16/f32/bf16/i32 and the
legacy quants also inherited the ne0 % QK_K == 0 gate and any row size
that is not a multiple of 256 fell back to the scheduler. Split the
cascade: unconditional support is restored everywhere, the gate stays
only on iq4_nl and mxfp4 whose 32-value sub-blocks do not guarantee the
QK_K super-blocks the kernel iterates on.