Make MLPBlock compatible with diffusion models

[Warvito]

Is your feature request related to a problem? Please describe.
To be able to re-use the MLPBlock in the implementation of diffusion models (developed in MONAI Generative Models (related to Project-MONAI/GenerativeModels#141)), it is necessary to be able to use the GEGLU activation function (from https://arxiv.org/pdf/2002.05202.pdf) in the block.

If GEGLU is selected, it would also be necessary to change self.linear1 to nn.Linear(hidden_size, mlp_dim* 2).

Describe the solution you'd like
Some implementations create a GEGLU block as a separated class and instantiate it inside the MLP block if this activation is selected, for example:

        project_in = nn.Sequential(
            nn.Linear(dim, inner_dim),
            nn.GELU()
        ) if not glu else GEGLU(dim, inner_dim)

from https://github.com/Stability-AI/stablediffusion/blob/8bde0cf64f3735bb33d93bdb8e28120be45c479b/ldm/modules/attention.py#L67 and https://github.com/huggingface/diffusers/blob/21bbc633c4d7b9bb7f74caf4b248c6a4079a85c6/src/diffusers/models/attention.py#L306 .
Maybe MONAI could use something similar.

[wyli]

thanks @Warvito, do you have time to submit a pull request for this?

[f-schnabel]

Hi, can I work on this issue if Warvito doesn't have time to implement this?

[Nic-Ma]

@Shadow-Devil Welcome your contribution!