Update for MXNet 2 Alpha

[leezu]

This switches doc and CI to MXNet 2 Alpha release

[leezu]

@barry-jin I only switched the AWS Batch tests to MXNet 2 Alpha, but the Github Actions tests are all failing. I think there's a regression with the recent FFI work.

[leezu]

�[0;31mMXNetError�[0m: Traceback (most recent call last):
  File "../include/mxnet/runtime/packed_func.h", line 416
MXNetError: Check failed: type_code_ == kDLFloat (4 vs. 2) : expected float but get NULL
MXNetError: Traceback (most recent call last):
  File "../include/mxnet/runtime/packed_func.h", line 416
MXNetError: Check failed: type_code_ == kDLFloat (4 vs. 2) : expected float but get NULL

[github-actions]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1546/19de74c2b03f22dde8311a0225b4571c2deef0e4/index.html