updated _forward_unim. to include descriptive error

[zikpefu]

Fixes #74303

Added error description for an unimplemented forward function.

Using torch summary to test the functionality
Before:

After:

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[albanD]

thanks!

[albanD]

@pytorchbot merge this please

[github-actions]

Hey @zikpefu.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.

[zikpefu]

Hey @albanD are there examples where I can get the release notes so that it doesn't get closed again? Thanks!

[albanD]

Hey!
Note that we don't use the github "merge" feature as we have a bit extra logic that runs. So our bot actually does the merge and closes the PR. This was commited in the main branch here: 43cc726

So this PR was merged and all is good! Thanks for taking the time to send it!

[zikpefu]

Ah I understand. Thanks for taking a look at it I appreciate it!