Pipeline can dynamic import task from arbitrary python file

[corey-nm]

Tested with:

• The new unit tests added to PR
• running deepsparse.server --task import:my_pipeline --model_path ... where my_pipeline.py had this contents

from deepsparse.pipeline import Pipeline
from deepsparse.transformers.pipelines.question_answering import (
    QuestionAnsweringPipeline,
)

TASK = "my_qa_task"
Pipeline.register(TASK)(QuestionAnsweringPipeline)

• running deepsparse.server --task import:deepsparse.my_pipeline --model_path ... from a different directory
• running deepsparse.server --config_file config.yaml where config.yaml had this contents:

models:
    - task: "import:my_pipeline"
      model_path: zoo:nlp/question_answering/bert-base/pytorch/huggingface/squad/base-none
      batch_size: 1
      alias: question_answering/base