SimMIM output num_channels should not be hardcoded
### Feature request

In all 3 simmim models,  reconstructed images channel is hardcoded as 3. This should be configurable as num_channels
```
deit/modeling_deit.py
swin/modeling_swin.py
vit/modeling_vit.py

nn.Conv2d(in_channels=config.hidden_size, out_channels=config.encoder_stride**2 * 3, kernel_size=1)
```
@NielsRogge 

### Motivation

I'm training a grayscale model, but the reconstructed image has different dimension as the input image.

### Your contribution

None