[WIP] Add support for elementwise unary ops (#77807)
* Add support for `log2, isinf, zeros_like`
* Add primitives for `log2` and `is_infinite`
I left a TODO to remove the `is_infinite` prim and to implement `isinf` reference using `isfinite` and `isnan`.
We're missing `real` and `imag` ops to handle complex tensors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77807
Approved by: https://github.com/mruberry
