Fix constexpr __host__ warning (#52702)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52702
Fixes:
```
stderr: caffe2/c10/util/MathConstants.h(22): warning: calling a constexpr __host__ function("from_bits") from a __host__ __device__ function("pi") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.
```
Test Plan: Sandcastle
Reviewed By: ngimel
Differential Revision: D26589533
fbshipit-source-id: 42c4b36b0ba1e08cbdc9a122fedf35610483c764
