[CI] Disable testing that downloads from mxnet

[Lunderberg]

The mxnet project was archived in 2023-09 (link). Downloads from the mxnet AWS
links (e.g. https://apache-mxnet.s3-accelerate.dualstack.amazonaws.com/gluon/models/resnet18_v1-a0666292.zip) no longer can be performed. This commit disables TVM CI steps that depend on downloading models from these locations.

[Lunderberg]

Failure to download from mxnet AWS links began in the evening of 2024-02-08, for multiple unrelated PRs. (example, example). Testing again this morning, but based on the archiving of the mxnet project, it may be permanent. If so, this commit can be used to avoid CI breakage.

[tqchen]

mxnet is getting deprecated, so we should remove the testcases/example that depends on it

[tqchen]

opened a related tracking issue #16547

[Lunderberg]

That was my guess, and agreed that it doesn't make sense to maintain integration with a deprecated framework.

Marking this PR as ready for review, so that it can unblock CI. It still attempts to download files, but should bail out on error rather than failing CI altogether. A follow-up PR can then remove the test cases altogether.

[yongwww]

good to have it to unblock the CI.

[Lunderberg]

Looks like CI is passing, except for the ARM tests, which are stalled waiting for ARM-GRAVITON3 instances. 1/8 has run since I checked this morning, so hopefully they'll get queued up sooner rather than later.

[yongwww]

arm tests are stuck #16549

[Lunderberg]

Thank you for mentioning that. Looks like the ARM CI was related to another of my PRs, which I recently rebased onto main. I've unstuck the ARM tests, and CI has now passed here.