Create Task base class

[jbischof]

This PR would be an replication of #621 for task models. Actually there is more to deduplicate in this case!

Steps:

• Create Task class in models/task.py, which inherits from PipelineModel
• Copy over following methods/properties to base class from BertClassifier:
  • preprocess_samples
  • backbone
  • preprocessor
  • from_config
  • presets (return {})
  • from_preset with string formatting params to adapt to preset and class names
• Remove these methods from each subclass and fill in the string params for from_preset.

Following #621, simplify the documentation for from_preset and move some of the examples to the generic constructor docstring.

[soma2000-lang]

@jbischof Can I work on this if someone is not already into it

[jbischof]

@soma2000-lang we're excited for new contributors but this task is taken. We will be adding more soon; we just emptied the queue before 0.4 release unfortunately!