Fix pow precision (#25476)
Summary:
Found in gpytorch:
```
test_computes_cubic_kernel (test.kernels.test_polynomial_kernel.TestPolynomialKernel) ... FAIL
======================================================================
FAIL: test_computes_cubic_kernel (test.kernels.test_polynomial_kernel.TestPolynomialKernel)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/mnt/xarfuse/uid-30041/0efc4638-seed-sandcastle-2ddc31a66f82cDbd-ns-4026533029/test/kernels/test_polynomial_kernel.py", line 70, in test_computes_cubic_kernel
self.assertLess(torch.norm(res - actual), 1e-5)
AssertionError: tensor(1.0790e-05, grad_fn=<NormBackward0>) not less than 1e-05
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25476
Differential Revision: D17147518
Pulled By: pbelevich
fbshipit-source-id: 60b619f5166d2bfaed7aa4803672e6be17d32b76
