changes to DL framework installation (amended) [skip ci]

conda activate deepchem

```

     2.1. DeepChem provides backend support with tensorflow, pytorch or jax.

For developemnt of DeepChem with one of the above backend, update the environment using

```bash

conda env update -n deepchem --file requirements/backend-name/env_backend-name.yml --prune

```

You need to replace `backend-name` with one of `tensorflow`, `pytorch` or `jax`. PyTorch

by default is installed with GPU support. For pytorch CPU, use the following command:

```bash

conda env update -n deepchem --file requirements/torch/env_torch.cpu.yml --prune

```

     2.1. DeepChem provides backend support for deep

learning using tensorflow, pytorch or jax. By default, the deep learning frameworks

gets installed in CPU. If GPU support is required, make sure CUDA is installed

and then install the desired deep learning framework before installing DeepChem:

- [tensorflow](https://www.tensorflow.org/install)

- [pytorch](https://pytorch.org/get-started/locally/#start-locally)

- [jax](https://github.com/google/jax#installation)

3. Install DeepChem in `develop` mode