Metapackage update for pytorch 1.4.0

[1297rohit]

No description provided.

[rohitgeo]

@cleebp or @scw - can you merge this PR? It adds the links for metapackages for the DL backbones.

[scw]

@rohitgeo this needs updating to have it compatible across releases so the older links are maintained for users not on the latest release — I will take a look.

[cleebp]

I think the changes Shaun requested were around hosting the older links for all versions of Pro and Server, currently the organization is difficult to understand as a user from a pro/server version standpoint.

Previously this section was organized like:

Pro versions | Server versions | Backbones package
-|-|-
2.9—3.2 | 10.9.1—11.2 | [PyTorch 1.8 `arcgis_dl_backbones` package](https://www.arcgis.com/home/item.html?id=ad8ecd2a45af4e3282a4d9ff455024be)
2.5—2.8 | 10.7.1—10.9.0 | [PyTorch 1.4 `arcgis_dl_backbones` package](https://www.arcgis.com/home/item.html?id=d404fd50d05d475f8d92eedb78e1c961)

Is it possible to return to an organization based on pro/server version instead of just arcgis/pytorch versions so users looking for links for versions like 2.9 and 10.9.1 (and older) can find their correct downloads? Still fine to list the pytorch versions, but generally users know their product version they are looking for and not the arcgis/pytorch version.