Addressing new pip syntax and dockerfile

Please install tensorflow v2.4.* before installing deepchem.

```bash

pip install tensorflow==2.4.*

pip install tensorflow~=2.4

```

Then, you install deepchem via pip or conda.  

    conda create -y --name deepchem python=3.6 && \

    . /miniconda/etc/profile.d/conda.sh && \

    conda activate deepchem && \

    pip install tensorflow==2.3.* deepchem && \

    pip install tensorflow~=2.4 deepchem && \

    conda install -c conda-forge rdkit && \

    conda clean -afy && \

    rm -rf ~/.cache/pip

    - pyGPGO

    - pymatgen==2020.12.31

    - simdna

    - tensorflow==2.4.*

    - tensorflow~=2.4

    - tensorflow_probability==0.12.*

    - torch-geometric==1.6.*

    - transformers==3.5.*

1. Create and merge a release PR

    - Modify the version in `deepchem/__init__.py` (Remove `.dev`, e.g. `2.4.0.dev` -> `2.4.0`)

    - Update the documents for installing a new package in `README.md` and `docs`

    - Update the dockerfile at `deepchem/docker/tag/Dockerfile`

2. Push a new tag to the merge commit -> release new PyPI package and docker image

3. Create and merge a release PR in the [feedstock repository](https://github.com/conda-forge/deepchem-feedstock) -> release new Conda Forge package

4. Publish the documents for a new tag in [ReadTheDocs](https://readthedocs.org/projects/deepchem/versions/).