[mlir][taco] Use sparse_tensor.out to write sparse tensors to files.

Add a Python method, output_sparse_tensor, to use sparse_tensor.out to write
a sparse tensor value to a file.

Modify the method that evaluates a tensor expression to return a pointer of the
MLIR sparse tensor for the result to delay the extraction of the coordinates and
non-zero values.

Implement the Tensor to_file method to evaluate the tensor assignment and write
the result to a file.

Add unit tests. Modify test golden files to reflect the change that TNS outputs
now have a comment line and two meta data lines.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D118956

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