Beta function (#78031)
Euler beta function:
```Python
torch.special.beta(input, other, *, out=None) → Tensor
```
`reentrant_gamma` and `reentrant_ln_gamma` implementations (using Stirling’s approximation) are provided. I started working on this before I realized we were missing a gamma implementation (despite providing incomplete gamma implementations). Uses the coefficients computed by Steve Moshier to replicate SciPy’s implementation. Likewise, it mimics SciPy’s behavior (instead of the behavior in Cephes).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78031
Approved by: https://github.com/mruberry
