Thanks to dear God, after hours of debugging I finally found out the cause:
The error messages with the form of :

While copying the parameter named “xxx.weight”, whose
dimensions in the model are torch.Size([yyy]) and whose
dimensions in the checkpoint are torch.Size([yyy]).

are actually generic messages, only returned when an exception has occured while copying the parameters in question.
Pytorch developers could easily, add the actual exception args into this spurious yet unhelpful message, so it could actually help better debug the issue at hand.
Anyway, looking at the exception which was by the way :
"copy_" not implemented for \'QInt8' 

you’ll now know what the actual issue is/was!