ggml-metal : fix padding in timestep embedding kernel

This commit fixes the zero padding for odd dimensions in
the timestep embedding kernel similar to the fix that was applied to the
cpu backend in Commit 9de447d
("ggml-cpu : fix padding in ggml_timestep_embedding (#15917)").

The motivation for this is that currently if an odd dimension is used,
the padding check incorrectly uses the dimension value for indexing.

For example, with dim=15:
Elements 0-6 are set to cosine values
Elements 7-13 are set to sine values
Element 14 is left uninitialized (contains garbage)
Element 15 is correctly set to zero

This fix changes embed_data[dim] to embed_data[2 * half] so that
element 14 (the first unused element) is properly set to zero as well
as the last element.

Author: danbev

ggml/src/ggml-metal/ggml-metal.metal

@@ -4167,6 +4167,7 @@ kernel void kernel_timestep_embedding_f32(
     }
 
     if (args.dim % 2 != 0 && tpitg.x == 0) {
+        embed_data[2 * half_] = 0.f;
         embed_data[args.dim] = 0.f;
     }
 }