Fix warnings coming from bernoulli and dirichlet kernels (#19933)
Summary:
Fixes the following warnings in CI:
```
/tmp/pip-req-build-un327jey/aten/src/ATen/native/cuda/Distributions.cu(123): warning: pointless comparison of unsigned integer with zero
detected during instantiation of "void <unnamed>::bernoulli_tensor_cuda_kernel<scalar_t,prob_t>(at::Tensor &, const at::Tensor &, std::pair<uint64_t, uint64_t>) [with scalar_t=uint8_t, prob_t=uint8_t]"
(238): here
```
```
/tmp/pip-req-build-un327jey/aten/src/ATen/native/cuda/Distributions.cu:220:116: warning: 'c10::ScalarType detail::scalar_type(const at::Type&)' is deprecated [-Wdeprecated-declarations]
/tmp/pip-req-build-un327jey/aten/src/ATen/Dispatch.h:46:1: note: declared here
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19933
Differential Revision: D15218177
Pulled By: ezyang
fbshipit-source-id: 3994f7130da1ab04d2e8c3c4fe97f722b7502b41
