Add SigOpt HPO to transformers trainer api
Author: kding1
Pull request: yes

# What does this PR do?
This PR adds SigOpt (https://sigopt.com/) hyper-parameter optimization support to transformer trainer api.

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@mfuntowicz 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
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- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
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Comments:
@LysandreJik this is because the sigopt token is not set properly in your test environment. Below are the steps:

- signup on sigopt HPO at https://app.sigopt.com/signup, it's free for everyone for the basic plan.
- login to your account, get your sigopt api token at your portal, and expose it in your environment variable with `export SIGOPT_API_TOKEN=your_sigopt_token_string`
- in your account portal, create a project named `huggingface`.

Once the training starts with sigopt HPO, you will get the real-time dashboard like this: https://app.sigopt.com/guest?guest_token=NJTRFVEPPGWTKZPLISXVWUXPIOFATREJOVAILZQBPSLOECMU

I have run the unit test with the refactor patch i just submitted and it works:
`$ pytest tests/test_trainer.py::TrainerHyperParameterSigOptIntegrationTest::test_hyperparameter_search`

Please let me know if there is any issue in your side.

rebase to fix conflict on dependency package.