Hotfix for TensorFlow 2.4, grouped allreduce updates

PyTorch 1.8 compatibility, Spark on Gloo stdout forwarding, bugfixes

Bugfixes for Ray, Databricks, and Spark Estimators

Local gradient aggregation, grouped allreduce

Elastic Horovod on Ray

Hotfix: build without MXNet installed

Bugfixes, Databricks support for Estimators

Elastic Horovod, Ray integration, CMake build, all-to-all op, gradien…

…t predivide factor

Hotfix to add PYTHONPATH to mpirun env

Hotfix for sync batch norm in PyTorch 1.5, mixed precision in TensorF…

…low 2.2