Move ax-platform to be installed by conda rather than pip

[jemissik]

ax-platform being installed by pip causes issues: e.g., if we pin pytorch, this might be ignored since pip will install a different version when ax-platform is installed. It will be much better to move ax-platform to be installed though conda.

Currently, the latest version of ax-platform isn't available through conda, so probably we want to wait until this is fixed. Ax lists adding a conda distribution in their wishlist facebook/Ax#566

[jemissik]

See facebook/Ax#1194