Robust stop criteria for fitting Platt transform

[jiong-zhang]

Issue #, if available: N/A

Description of changes:

• Add new stopping criteria for Platt transform. The original stopping criteria, i.e. $|grad| < \epsilon$, might have numerical stability issue and lead to line search failure when the curvature is large ($|\mathcal{H}|\rightarrow \inf, dA \rightarrow 0, dB \rightarrow 0$).
• Allow user specified max steps and $\epsilon$ in fit_platt_transform

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