RFC: (Optionally?) hide gin from tracebacks

Right now wrapping a function with gin adds two lines to every traceback. For projects which use gin heavily, this gets to be...kind of a lot, see example at the bottom of this bug report.

Would you be willing to take a patch which (optionally? always?) hides gin from stack traces?

(I'm no Python expert, but I think it's possible to do this, see e.g. https://stackoverflow.com/questions/23765707/python-2-and-3-compatible-way-of-hiding-a-decorator-from-stacktrace)

Example of highly-decorated stack trace:

```
Traceback (most recent call last):
File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/trainer.py", line 133, in
app.run(main)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/absl/app.py", line 300, in run
_run_main(main, args)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/absl/app.py", line 251, in _run_main
sys.exit(main(argv))
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/trainer.py", line 129, in main
trax.train(output_dir=output_dir)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1073, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/utils.py", line 49, in augment_exception_message_and_reraise
six.raise_from(proxy.with_traceback(exception.__traceback__), None)
File "", line 3, in raise_from
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1050, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/trax.py", line 878, in train
save_steps=save_steps, has_weights=has_weights)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1073, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/utils.py", line 49, in augment_exception_message_and_reraise
six.raise_from(proxy.with_traceback(exception.__traceback__), None)
File "", line 3, in raise_from
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1050, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/trax.py", line 540, in __init__
inputs = inputs(n_devices)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1073, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/utils.py", line 49, in augment_exception_message_and_reraise
six.raise_from(proxy.with_traceback(exception.__traceback__), None)
File "", line 3, in raise_from
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1050, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/inputs.py", line 89, in inputs
dataset_name, data_dir, input_name, n_devices)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/inputs.py", line 527, in _train_and_eval_batches
dataset, data_dir)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1073, in gin_wrapper
utils.augment_exception_message_and_reraise(e, err_str)
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/utils.py", line 49, in augment_exception_message_and_reraise
six.raise_from(proxy.with_traceback(exception.__traceback__), None)
File "", line 3, in raise_from
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/gin/config.py", line 1050, in gin_wrapper
return fn(*new_args, **new_kwargs)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/inputs.py", line 217, in train_and_eval_dataset
return _train_and_eval_dataset_v1(dataset_name[4:], data_dir)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/trax/inputs.py", line 280, in _train_and_eval_dataset_v1
train_dataset = problem.dataset(tf.estimator.ModeKeys.TRAIN, data_dir)
File "/usr/local/google/home/jlebar/code/tensor2tensor/tensor2tensor/data_generators/problem.py", line 646, in dataset
data_filepattern))
File "/usr/local/google/home/jlebar/.local/lib/python3.6/site-packages/tensorflow/contrib/slim/python/slim/data/parallel_reader.py", line 316, in get_data_files
raise ValueError('No data files found in %s' % (data_sources,))
```