Fix(Early Stopping): move best score to device

[kaushikb11]

What does this PR do?

Fixes #7936

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[codecov]

Codecov Report

Merging #7959 (c5c4c76) into master (599d6db) will decrease coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7959    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         207     207            
  Lines       13484   13484            
=======================================
- Hits        12354   11713   -641     
- Misses       1130    1771   +641     

[ethanwharris]

LGTM - at least, it does what it says it does haha

[tchaton]

LGTM !

[awaelchli]

Suggested change

	- Fixed move best score to device in EarlyStopping Callback ([#7959](https://github.com/PyTorchLightning/pytorch-lightning/pull/7959))
	- Fixed move best score to device in EarlyStopping Callback ([#7959](https://github.com/PyTorchLightning/pytorch-lightning/pull/7959))

as your comments suggest, this fix applies only for tpu right?

[kaushikb11]

It applies to other accelerators as well.

[awaelchli]

how would I reproduce an error on a GPU for example?

why can't we do for example this:

self.best_score.to(current.device)

moving one tensor to the device but the other not in a monitor_op is only going to raise questions for the reader of this code

[kaushikb11]

>>> torch_inf = torch.tensor(np.Inf)
>>> value = torch.tensor(5, device=xm.xla_device())
>>> torch.lt(value, torch_inf)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: torch_xla/csrc/aten_xla_bridge.cpp:69 : Check failed: xtensor
*** Begin stack trace ***
	tensorflow::CurrentStackTrace()

Similar code for Cuda devices won't throw an error.

[sidml]

I got the following error (device with 2 gpus and using ddp)
RuntimeError: Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu!`

I also felt that self.best_score.to(current.device) was safer..