Released Chainer v.6.6.0

[nis...@preferred.jp]

We have released Chainer v.6.6.0, the release note is:

This is the release note of v6.6.0. See here for the complete list of solved issues and merged PRs.

Bug Fixes

• Fix SCE with ChainerX and normalize (#8311)
• Fix kernel of double backward of max_pooling_2d (#8329)
• Fix ChainerX fallback condition in batch normalization (#8368)
• Fix optimizer_hooks.GradientHardClipping for scalar array (#8372)
• Fix pickling of optimizers (#8417)
• Register uninitialized persistents (#8446)

Enhancements

• Compute F.negative_sampling in fp32 for fp16 inputs (#8309)
• Fix optimizer_hooks.GradientHardClipping for ChainerX (#8377, thanks @kshitij12345!)

Documentation

• Fix documentation of NStepGRUBase (#8337, thanks @msakai!)
• Fix n-step RNN docs (#8402)
• Fix typo in /examples/seq2seq/README.md (#8404, thanks @tanaken0515!)
• Changes citation to new KDD paper (#8418)
• Link to examples directory for the current branch (#8423)
• Update expected messages of type_check errors (#8456)
• Update requirements (#8502)

Tests

• Fix Decorrelated Batch Normalization tests (#8340)
• Add missing FlexCI configurations (#8352)
• Use LinkTestCase for L.GroupNormalization (#8355)
• Show pytest summary in flexCI (#8369)
• Set CHAINER_CI in Travis CI (#8373)
• Set CHAINER_CI in ChainerX tests in Jenkins (#8375)
• Set CHAINER_CI in Chainer tests in FlexCI (#8381)
• Print installed packages in pytest (#8386)
• Print actual array values in FunctionTest modified input error (#8388)
• Avoid non-differential point in TestTriplet (#8396)
• Use different docker image for each base development branch (#8401)
• Disable ChainerMN FlexCI tests on v6 (#8411)
• Use fix_random in xfail backward tests (#8457)
• Avoid ChainerX slow tests in Jenkins (#8474)
• Use CuPy v6 in ChainerX test in Jenkins (#8477)
• Skip some Convolution2D tests for older numpy versions (#8478)
• Fix Travis Openssl Error in OSX (#8480)
• Fix flaky test of _modified_xlogx (#8486)
• Add error message for invalid base branch in pfnCI (#8500)
• Adjust timeout and build memory usage in FlexCI (#8503)