Use DS callable API to allow hf_scheduler + ds_optimizer
This PR:

- Used the (new) Callable api of deepspeed.initialize() to enable combining hf schedulers with deepspeed optimizers.  
- `create_scheduler` now has an optional `optimizer` arg
- Updates relevant unit test.

Blocking events: All unblocked now.

- [x] depends on deepspeed PR [1316](https://github.com/microsoft/DeepSpeed/pull/1316).
- [x] needs new deepspeed version after PR is merged and need to update the dependencies when that happens.

deepspeed: @stas00. 

* [x] https://github.com/microsoft/DeepSpeed/pull/1316 is merged
* [x] v0.5.1 released to PyPI: https://pypi.org/project/deepspeed/0.5.1/