loadnifti transform and common dataset

[Nic-Ma]

Is your feature request related to a problem? Please describe.

1. Now we only have a NiftiDataset which is not compatible with all kinds of tasks.
2. The "load_nifti" can be a transform instead of a special function for unified usage.

Describe the solution you'd like

1. Develop array level and dict level transforms for "load_nifti" feature.
2. Develop a common Dataset for a list of training data in dict format.
3. Develop a 3D medical image segmentation example based on above solution.