136 3d classification example

[Nic-Ma]

Fixes #136 .

Description

This PR mainly contains the below features:

1. Temporarily modify NiftiDataset to support classification tasks.
2. Developed a 3D classification example based on temp dataset and DenseNet3D network.

Next Steps:

1. @wyli will help to replace the dataset with a suitable public 3D classification dataset.
2. I will try to refactor "load_nifti" as a transform and change Dataset class into a common Dataset.
   Will submit another PR because this will be a big change.

Status

Work in progress

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or new feature that would cause existing functionality to change)
• New tests added to cover the changes
• Docstrings/Documentation updated

[wyli]

if this is the final version of nifti_dataset, would be great to include a unit test; otherwise it looks good to me