Reander problem on \frac \right in some tutorials file

[DrDavidS]

https://pytorch.org/tutorials/beginner/examples_autograd/polynomial_custom_function.html#sphx-glr-beginner-examples-autograd-polynomial-custom-function-py

Here in the "where (P_3(x)= rac{1}{2}\left(5x^3-3x ight))... ", \f in \frac and \r in \right has gone~ so the function can't be rendered correctly.

Also in "By mathematics, (P_3'(x)= rac{3}{2}\left(5x^2-1 ight))..."

But in https://pytorch.org/tutorials/beginner/pytorch_with_examples.html#pytorch-defining-new-autograd-functions it seems good. I don't know why...

[svekars]

Fixed in https://github.com/pytorch/tutorials/pull/1881.