How could this possibly be forward ?

“save with 1.5” -> “load with 1.6” is backward compat: files created with older version can still be loaded with newer versions.
“save with 1.6” -> “load with 1.5” is forward compat: the older version needs to know how to load newer files.

After all, all you need to do to make the saved weight backward compatible is just change a letter.

Yes for your particular case it just works. But depending on what you save, it might fail unpredictably.
Since 99% of people will never need forward compatibility, we did not added the complexity in the code to make this work, maintain it and test it.

The latest pip and conda I found is also 1.5,

You can select the nightly build on the get started page: https://pytorch.org/get-started/locally/ 10 to get a more recent version without having to compile yourself. These are the same ones that xla uses IIRC.