Should ConcatItemsd require at least 2 keys?

[function2-llx]

Hi everyone I'm using ConcatItemsd for concatenating the inputs from multi-modalities images. During development sometimes I adjust the number of modalities of inputs, even to single modality. However I found that ConcatItemsd in monai currently does not allow this, with an exception raised here.

This restriction is inconvenient for me and personally I can't find the point of this restriction because both numpy.concatenate and torch.cat work fine with an input list of length 1. I'd like to hear your opinions on this :)

[wyli]

makes sense, I'm converting this into a feature request, thank you!

#1881

[function2-llx]

Thanks so much!