[MRG] FIX LMNN gradient and cost function#201

Merged

perimosocordiae merged 8 commits intoscikit-learn-contrib:masterfrom

wdevazelhes:fix/LMNN_grad

May 29, 2019

Commits on May 13, 2019

TST: make tests for LMNN gradient
William de Vazelhes
committed

Commits on May 21, 2019

FIX: fix gradient computation
William de Vazelhes
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Simplify expression
William de Vazelhes
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Be more tolerant for checking NCA
William de Vazelhes
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Address https://github.com/metric-learn/metric-learn/pull/201#discussion_r286032898
William de Vazelhes
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Commits on May 22, 2019

Add checks for bounds argument
William de Vazelhes
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Revert "Add checks for bounds argument"
William de Vazelhes
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Add missing return
William de Vazelhes
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