[mlir][sparse] refine trait of sparse_tensor.convert

Rationale:
The currently used trait was demanding that all types are the same
which is not true (since the sparse part may change and the dim sizes
may be relaxed). This revision uses the correct trait and makes the
rank match test explicit in the verify method.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D112576