Drop rc1 to do a proper release.

Fix policy softmax accuracy if masking is enabled. (LeelaChessZero#912)

* Do softmax outside backend on set of legal moves.

* Remove policy softmax from blas backend.

* Remove policy softmax from CUDA backend.

* Remove policy softmax from OpenCL backend.

* Remove policy softmax from TensorFlow backend.

* Use FastExp for policy softmax calculations.

* Fix for negative exponentials.

* Revert "Fix for negative exponentials."

This reverts commit 9fb73d0.

* Fuse softmax with softmax temperature.

* Modify random backend policy value distribution.

* Comment improvements.

Bump version.

Bump version to 0.21.3

Bump version to 0.21.1