Apache Hadoop (HDFS) File Loading from_pretrained
# 🚀 Feature request

Loading configuration file for **transformers.AutoConfig** and **transformers.AutoModelForSequenceClassification** using the function **from_pretrained** by giving the HDFS file path

## Motivation
In case of file that is not locally available, the library utilizes the **get_from_cache** function inside **transformers/file_utils.py** file to try to download the model from the remote resource. But, in case of no ETag being present in the header of the response returned, an OSError("Distant resource does not have an ETag, we won't be able to reliably ensure reproducibility.") exception is raised. In case of HDFS, this ETag validation shouldn't be considered as a mandatory requirement but as an optional one. Either another mechanism should be used to ensure the reliability of the resource or this ETag check should be made optional.
Please see below the code-snippet screenshot of the aforementioned file.
![image](https://user-images.githubusercontent.com/44267622/103738038-262dc080-5015-11eb-98f4-047be7b81bdd.png)


Additional information.
Apache Hadoop Version: 2.7.7, rc1aad84bd27cd79c3d1a7dd58202a8c3ee1ed3ac

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