Additional command line argument --clean for clean installation.

[Anindyadeep]

Consider this edge case:

1. A user does a benchmark with the CPU first
2. Now they want to do it in Mac or CUDA. But suppose CUDA/Mac has a different installation procedure.

But since they already did a benchmark initially on CPU, so they have do delete the venv first and then do the benchmarking once again. Instead a quick solution would be to add an argument --clean which will do a new clean installation by first removing existing venv. In this way there would be no conflicts.