Allow to run multiple trials on one GPU

[squirrelsc]

What would you like to be added:
Add some configurations that user can decide how many trials can be run on same GPU.
For example,
MaxMemOnGPU: 2G -- user to declare how many mem of GPU is used in on trial. It could be used to
MaxTrialsNumOnGPU: 3
EnableMultipleTrialsOnGPU: true

Why is this needed: It can improve utilization in some cases, that GPU mem is free a lot, and there is not a single bottleneck on IO or computing.

Without this feature, how does current nni work：
schedule 1 trial on one GPU.

Components that may involve changes:

Brief description of your proposal if any:

[leelaylay]

Similar to issue #608 #775

[scarlett2018]

#775 was released in 0.7