[FEATURE] Feature only support for Twins-PVT and Mvitv2

[L-Reichardt]

Both are pyramid networks and can be used for multi-scale feature extraction, but to my knowledge do not support it like similar architectures such as PVT or Swin.

[rwightman]

@L-Reichardt the efficient mechanism for feature extraction relies on sequential stack at the stage level of the pyramid network, many pure vit / vit-hybrid need nn.ModuleList (and have extra args) or have extra root level modules in the model that can't be sequentialized... I have a very rough draft for another approach that'd address these but have another project in the way right now...