LSF support, Spark on Gloo, bugfixes and performance improvements

TensorFlow 2.2 compatibility, framework install deps, MPI args for ho…

…rovodrun, bugfixes

Horovod Spark Estimators, Spark 3 support, Join operation, and Intera…

…ctive Run mode for notebooks

Bugfixes for MPI, Gloo, and TF Keras

Bugfix: autotune works with horovodrun, added log level arguments to …

…horovodrun

Horovodrun config file, TF: switch allreduce back to compute_gradient…

…s(), bugfixes

Bugfix release

Bugfix: make Horovodrun work with ABI-incompatible frameworks

MPI-less Horovod alpha, TensorFlow 2.0 support, MXNet 1.5.0 support, …

…g++ autodetection, RDMA in Dockerfile

Improved optimizer.skip_synchronize() API for NVIDIA AMP