[TOPI] Fix conv2d transpose for small channel

[masahi]

Fixes the issue reported in https://discuss.tvm.apache.org/t/model-compilation-fails-on-gpu-target/13866

For certain conv2d_transpose workloads, we get funny allocations like

 allocate data_pad[float32 * ((floordiv(((blockIdx.y*2) + 1), 3)*4608) + 576)], storage_scope = shared
 allocate p1.shared[float32 * ((floordiv(((blockIdx.y*2) + 1), 3)*12288) + 1536)], storage_scope = shared

This is apparently due to trying to split a small channel by a large factor.

cc @vinx13

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[junrushao]

LGTM! Thanks for the fix!