Trial jobs need to get a unique ID value

[chicm-ms]

For example, it is a common practice to do k fold data split to improve model performance. In order to do k fold split, each of my trial jobs need to train on a unique split, to do this, my trial jobs need to get a unique ID value.
In short, trial jobs need to get an Id if they need to corporate on something.
There are two possible solutions to implement this:

1. Use search space, need to add new type, such as sequence, can be implemented in either builtin or customized Tuner.
2. Add an API, such as nni.get_trial_id()

[yds05]

We need to provide a new API in SDK, like nni.get_sequence_num(), enable NNI trial identify it's unique auto-incremented sequence ID(start from 0. e.g)

[chicm-ms]

Done,
#203