Trying to work around it with a ByteTensor as dst and ndarray of uint8 as follows:
def copy_tensor(dst, src):
    for z in range(Z_MAX):
        for y in range(Y_MAX):
            for x in range(X_MAX):
                val = src[z][y][x]
                dst[z][y][x] = val

yields:
RuntimeError: can’t assign a numpy.uint8 to a scalar value of type int
The labels masks are 192x160x192, so it really would be nicer to keep them as single byte tensors.