Fix known issue of job context using.

[BalaBalaYi]

What changes were proposed in this pull request?

1. Rename the function in JobContext if the method can read and write the core 'nodes' object.
2. Other fixed.

Why are the changes needed?

To fix known issue.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

UT and training with failover(doing).

[codecov]

Codecov Report

Attention: Patch coverage is 86.53846% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.64%. Comparing base (60bdbd1) to head (131e5c4).

Files with missing lines	Patch %	Lines
dlrover/python/master/node/job_context.py	33.33%	6 Missing ⚠️
dlrover/python/master/node/training_node.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1326      +/-   ##
==========================================
- Coverage   80.68%   80.64%   -0.04%     
==========================================
  Files         225      225              
  Lines       21262    21267       +5     
==========================================
- Hits        17155    17151       -4     
- Misses       4107     4116       +9     

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