Write sparse input to dense output all at once

[drhagen]

Right now, if in a given iteration and the input is sparse while the output is dense, the iteration loop is dense and the loop writes each zero in between the sparse values. This is particularly inefficient when doing compute on an already assembled output. It also complicates the code having to always write to the output even when the input is sparse.

When the output is dense and the input is sparse, Tensora should preallocate the output and the iteration should be sparse. Care needs to be taken when subsequent layers are sparse. This might be less efficient when doing evaluate, but perhaps not depending on how optimized the processor is for writing blocks of zeros to memory. This preallocation only needs to be done during assembly and not during compute.