Update install links for PyTorch XLA in MNIST TPU example

[Ishan-Kumar2]

The PyTorch XLA install link in MNIST_on_TPU is outdated and doesn't work anymore, this can be updated to the 'nightly' version or getting the latest version from XLA repo as done in this example.

[vfdev-5]

@Ishan-Kumar2 thanks for reporting. Yes, using nightly or latest stable (as here) can be a fix.

[FarehaNousheen]

Hi @vfdev-5 @Ishan-Kumar2 the version needs to be updated as shown in the example in this issue. Kindly assign this to me.