add checkAlive in NasBertTrainer

[LittleLittleCloud]

We are excited to review your PR.

So we can do the best job, please check:

• There's a descriptive title that will make sense to other developers some time from now.
• There's associated issues. All PR's should have issue(s) associated - unless a trivial self-evident change such as fixing a typo. You can use the format Fixes #nnnn in your description to cause GitHub to automatically close the issue(s) when your PR is merged.
• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

#6525

[codecov]

Codecov Report

Merging #6546 (81c76b5) into main (eeba2ee) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6546      +/-   ##
==========================================
- Coverage   68.39%   68.39%   -0.01%     
==========================================
  Files        1174     1174              
  Lines      248049   248054       +5     
  Branches    25909    25909              
==========================================
- Hits       169661   169652       -9     
- Misses      71614    71634      +20     
+ Partials     6774     6768       -6     

Flag	Coverage Δ
Debug	68.39% <100.00%> (-0.01%)	⬇️
production	62.76% <100.00%> (-0.01%)	⬇️
test	88.93% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../Microsoft.ML.TorchSharp/NasBert/NasBertTrainer.cs	83.64% <100.00%> (+0.17%)	⬆️
src/Microsoft.ML.Core/Data/ProgressReporter.cs	70.95% <0.00%> (-6.99%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	89.61% <0.00%> (-0.16%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.53% <0.00%> (+0.14%)	⬆️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	86.87% <0.00%> (+1.13%)	⬆️