Update rcp_checker.py to handle llama31_8b epochs correctly

[gyulaz-htec]

Right now the RCP check fails for llama3 using the reference implementation. It throws an error because it trries to get the epch from epoch_nums instead of sample_count:

  File "/home/gyuzakor/actions-runner/_work/_tool/Python/3.12.11/x64/lib/python3.12/site-packages/mlperf_logging/rcp_checker/rcp_checker.py", line 496, in check_directory
    bs, subm_epochs, benchmark = get_submission_epochs(result_files, version, bert_train_samples)
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/gyuzakor/actions-runner/_work/_tool/Python/3.12.11/x64/lib/python3.12/site-packages/mlperf_logging/rcp_checker/rcp_checker.py", line 136, in get_submission_epochs
    curr_not_converged, curr_subm_epochs, curr_bs, curr_benchmark = read_submission_file(
                                                                    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/gyuzakor/actions-runner/_work/_tool/Python/3.12.11/x64/lib/python3.12/site-packages/mlperf_logging/rcp_checker/rcp_checker.py", line 93, in read_submission_file
    conv_epoch = json.loads(eval_accuracy_str)["metadata"]["epoch_num"]
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^

Processing llama31_8b logs the same way as llama31_405b logs should solve the problem.

[github-actions]

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[gyulaz-htec]

recheck

[ShriyaRishab]

@gyulaz-htec - can you please fix the CLA error so we can merge this?
Just follow these instructions if it is your time contributing to MLC repositories - #434 (comment)

[ShriyaRishab]

Closing as its a duplicate of #435