Ensure strides array initialised to zero

[jwallwork23]

While working on #236, I hit an overflow error in the 3D torch_tensor_from_array unit test. Digging into it, I noticed that the strides array in torch_tensor_from_blob is not initialised to zero!

I also switched tensor_shape and layout to be assumed shape as I don't think assumed size is necessary.

[jwallwork23]

@jatkinson1000 and I had a discussion with @SimonClifford and determined that the same assumed shape change should be made else where - see 190782a. It still needs to be assumed size in the C interfaces because assumed shape is not interoperable - see https://stackoverflow.com/questions/38349435/in-fortran2003-is-1d-assumed-shape-array-interoperable-with-c.