pipeline.from_pretrained
# 🚀 Feature request

Nearly everyone who is using the transformers library is aware of the `from_pretrained()` and `save_pretrained()` concept. The [Pipeline class](https://huggingface.co/transformers/main_classes/pipelines.html#parent-class-pipeline) is currently only providing the `save_pretrained()` method which can cause confusion for some users as saving and loading of the pipeline needs to be done like this:
```
from transformers import pipeline

TASK = 'something'
DIRECTORY='something'
classifier = pipeline(TASK)
classifier.save_pretrained(DIRECTORY)  
c2 = pipeline(task = TASK, model=DIRECTORY, tokenizer=DIRECTORY)
``` 
This is probably not that obvious for people who just read the documentation and not the code. I suggest implementing the `from_pretrained()` method for the pipelines to make the library even more intuitive. 

## Your contribution

There is actually not much to do since the tokenizer and model are loaded by the corresponding Auto classes. The only information that is missing to construct the saved pipeline with `pipeline.from_pretraind(PATH)` is the task identifier. This information should be stored in a new separate file called `pipeline_config.json`.

I can provide a PR if you think this is a useful enhancement for the library @LysandreJik



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