Yes, I am learning about semantic segmentation, too.
I read your and ptrblck’s replies,
and I think there might be an error as you said you had wanted a 1x64x64 image but got a 25x6x192x192 tensor.
I think ptrblck’s suggesstion is good, do the mask loading as:

Reading the image file;
Mapping the pixel value into the train class ID.