[mlir][sparse][taco] Support true dense tensors and all dense sparse tensors.

The only method to create a true dense tensor (i.e un-annotated) in MLIR-PyTACO
is through the from_array method. However, the annotated all dense tensors are
also implemented as true dense tensor currently. The PR fixes the
implementation to support annotated all dense sparse tensors.

Extend the tensor init method to support the construction of a tensor without
any sparsity annotation.

Change the tensor to_file method to only support writing unpacked sparse
tensors to file through the MLIR sparse tensor dialect.

Add unit tests for true dense tensors and all dense sparse tensors.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D119500