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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Description of changes:
fit_platt_transformBy submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.