* Add basic guides structure of Training - TensorFlow
* Add how-to guides: How to accelerate a TensorFlow Keras application on training workloads through multiple instances
* Change import order and add pip install for tensorflow-dataset
* Diable other nano tests for now
* Add github action tests for how-to guides Tensorflow training
* Use jupyter nbconvert to test notebooks for training tensorflow instead to avoid errors
* Add how-to guide: How to optimize your model with a sparse Embedding layer and SparseAdam optimizer
* Enable other nano tests again
* Small Revision: fix typos
* Small Revision: refactor some sentences
* Revision: refactor contents based on comments
* Add How-to guides: How to choose the number of processes for multi-instance training
* Small Revision: fix typos and refactor some sentences
* Make timeout time for github action longer for TensorFlow, 600s->700s
