add setter for dtype

[kaiyuyue]

Sometimes, we would like to just use the text encoder by removing the visual model, e.g.,

model, _ = clip.load("ViT-B/32")
del model.visual  # keep the text encoder only
text_features = model.encode_text(text_tokens)

This will throw an error that CLIP has no attribute self.dtype because the dtype attr depends on the visual model part. Also, we cannot directly assign a new dtype to it because it is decorated by the @property.

So this PR aims to solve the above issue. With this PR, we can do

model.dtype = ${new dtype} 

by adding a property setter for the attribute of dtype.