Add GRN, ConvNeXtBlock_V2 and UpConvNeXtBlock_V2 blocks

[anepaniagua]

I have included the necessary blocks to create the second version of the U-NeXt model (U-NeXt-V2):

• In blocks.py: I added a GRN (Global Response Normalization) block, as well as the ConvNeXtBlock_V2 and UpConvNeXtBlock_V2 blocks.