Update 'torch' extra dependency specifiers.

- Drop support for Torch 1.10-1.12, which complexify rules for functorch.
- Make "torch", "torch1" and "torch2" specifiers deal merely with Torch,
  leaving Opacus to the "dp" specifier, as should be.
- Have functorch be installed together with torch, removing some burden
  from our specifier, and enabling support for the new Torch 2.1.