Migrate x-as-service to jina 0.4.1

[bwanglzu]

This PR was aligned with issue #116 , migrate x-as-service example for Jina 0.4.1 release, includes:

1. Code migration based on migration guide.
2. Minor changes on Readme.

To run this example, you should build pytorch-transformer image locally using jina 0.4.1 with Dockerfile:

FROM pytorch/pytorch:1.4-cuda10.1-cudnn7-runtime

RUN pip install transformers --no-cache-dir --compile && \
    python -c "from transformers import *; x='bert-base-uncased'; BertModel.from_pretrained(x); BertTokenizer.from_pretrained(x)"

RUN pip install git+https://github.com/jina-ai/jina.git/@master

ENTRYPOINT ["jina", "pod", "--uses", "TransformerTorchEncoder"]

And command:

docker build -f Dockerfile -t jinaai/hub.executors.encoders.nlp.transformers-pytorch .

[nan-wang]

LGTM👍