Improve readability of the extra message errors in assertEqual (#87202)
Goes from (note the `linspace.default` is very difficult to find)
```
Mismatched elements: 15 / 50 (30.0%)
Greatest absolute difference: 1 at index (17,)
Greatest relative difference: 1.0 at index (17,) : linspace.default
args = (0, -3, 50)
kwargs = {'dtype': torch.int16, 'device': device(type='cpu'),
'pin_memory': False}
```
to
```
Mismatched elements: 15 / 50 (30.0%)
Greatest absolute difference: 1 at index (17,)
Greatest relative difference: 1.0 at index (17,)
linspace.default
args = (0, -3, 50)
kwargs = {'dtype': torch.int16, 'device': device(type='cpu'),
'pin_memory': False}
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87202
Approved by: https://github.com/ezyang
