* add optimizer.py
* fix style
* set version for mypy
* add thread control for quantize, support openvino pot
* update optimizer
* fix typo
* update workflow
* temp commit
* modify ut
* refactor optimizer
* rebase, update based on comments
* Add api docs for bigdl.nano.tf.keras.layers.Embedding and bigdl.nano.tf.optimizers.SparseAdam
* Update API document link in related how-to guide
* Remove non-related parameter explanation for Embedding
* Add param explanation for kwargs in for Embedding, and small revision
* Small fix