Log the timestamps of each prefill/decode iteration

[yuhsuan-t]

Motivation

Logging the timestamps along with other stats helps with debugging performance issues

Modifications

Add timestamps to the prefill/decode logging functions.

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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[xiezhq-hermann]

@hebiao064 can you take a final pass and we finalize the merge of this PR?

[Fridge003]

Hi @yuhsuan-t, please fix the conflict

[yuhsuan-t]

@Fridge003 Fixed the conflict. Thanks!

[xiezhq-hermann]

@yuhsuan-t sorry for the delay, I approved the change and could you help doing a rebase one more time and we can get it merged. Thanks!