Make normalization type in DenseNet configurable

[holgerroth]

Is your feature request related to a problem? Please describe.
BatchNorm is sometimes not optimal for certain applications, e.g. federated learning.

Describe the solution you'd like
Please make the normalization type in DenseNet implementation configurable to allow the use of other types of normalization apart from BatchNorm, like Instance, Group norm, etc.
See here

MONAI/monai/networks/nets/densenet.py

Line 152 in 03a8372

norm_type: Type[Union[nn.BatchNorm1d, nn.BatchNorm2d, nn.BatchNorm3d]] = Norm[Norm.BATCH, spatial_dims]

Describe alternatives you've considered
n/a

Additional context
n/a

[yiheng-wang-nv]

Mark: https://github.com/Project-MONAI/MONAI/blob/dev/monai/networks/nets/efficientnet.py also need to be changed for the same issue, I'll submit a PR for these two networks soon.