sycl: Improve mul_mat_id memory efficiency and add BF16 fast path (#22119)

* sycl: size mul_mat_id staging buffers by routed rows

Previously src1_contiguous/dst_contiguous in ggml_sycl_mul_mat_id were
sized to ggml_nelements(src1/dst), which over-allocates when ne12 > 1
and can fail with UR_RESULT_ERROR_OUT_OF_HOST_MEMORY on Level Zero for
MoE models (notably with --cpu-moe). Size them by the actual number of
routed rows (ids->ne[1] * n_ids) instead.

* sycl: add bf16 mul_mat fast path via DNNL

When src0 is BF16 (commonly the case for lm_head / output.weight), the
existing f16 path is skipped because bf16 isn't covered, and the f32
fallback dequantizes the entire src0 slab to f32 in a single pool alloc
(row_diff*ne00 floats). For large-vocab models this can reach several
GB and fail with UR_RESULT_ERROR_OUT_OF_HOST_MEMORY on Level Zero.

Add a bf16xbf16 -> f32 DNNL matmul fast path that uses the bf16 storage
in place and only materializes a small src1 bf16 conversion buffer. bf16
matmul accumulates in f32, so it's correct even when the op requests
GGML_PREC_F32 (as lm_head does).

- gemm.hpp: map bfloat16 to dnnl::memory::data_type::bf16.
- convert.{hpp,cpp}: expose ggml_get_to_bf16_sycl for f32/f16/bf16 -> bf16.
- ggml-sycl.cpp: take the bf16 path early in ggml_sycl_op_mul_mat_sycl
  when DNNL and GGML_SYCL_HAS_BF16 are both available.

ggml/src/ggml-sycl/set_rows.cpp

@@ -4,7 +4,11 @@
 namespace utils {
 template<typename T>
 static constexpr bool is_arithmetic_v() {
-    return std::is_arithmetic_v<T> || std::is_same_v<T, sycl::half> || std::is_same_v<T, sycl::ext::oneapi::bfloat16>;
+    return std::is_arithmetic_v<T> || std::is_same_v<T, sycl::half>
+#ifdef GGML_SYCL_HAS_BF16
+        || std::is_same_v<T, sycl::ext::oneapi::bfloat16>
+#endif
+        ;
 }
 }
 
@@ -181,6 +185,7 @@ static void set_rows_sycl(ggml_backend_sycl_context & ctx, const ggml_tensor * s
                 stream
             );
             break;
+#ifdef GGML_SYCL_HAS_BF16
         case GGML_TYPE_BF16:
             set_rows_sycl<TIn, TIdx, sycl::ext::oneapi::bfloat16>(
                 src0_d, src1_d, (char *)dst->data,
@@ -193,6 +198,7 @@ static void set_rows_sycl(ggml_backend_sycl_context & ctx, const ggml_tensor * s
                 stream
             );
             break;
+#endif
         case GGML_TYPE_Q8_0:
             set_rows_sycl_q<TIdx, block_q8_0, QK8_0, cpy_blck_f32_q8_0>(src0_d, src1_d, (block_q8_0 *)dst->data, ne00, ne01, ne02, ne03, ne10, ne11, ne12, ne13, nb00, nb01, nb02, nb03, nb10, nb11, nb12, nb13, nb1, nb2, nb3, stream);
             break;