docs(tuner): add loss function explain for tuner

[hanxiao]

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[codecov]

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@@           Coverage Diff           @@
##             main     #138   +/-   ##
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  Coverage        ?   78.51%           
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  Files           ?       24           
  Lines           ?     1159           
  Branches        ?        0           
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  Hits            ?      910           
  Misses          ?      249           
  Partials        ?        0           

Flag	Coverage Δ
finetuner	78.51% <0.00%> (?)

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