[mlir][tosa] Fix lowering of tosa.conv2d (#73240)

The lowering of tosa.conv2d produces an illegal tensor.empty operation
where the number of inputs do not match the number of dynamic dimensions
in the output type.

The fix is to base the generation of tensor.dim operations off the
result type of the conv2d operation, rather than the input type. The
problem and fix are very similar to this fix

https://github.com/llvm/llvm-project/pull/72724

but for convolution.