Fix Flash CLI: in case of converting functions to classes + JIT tracing tests

[krshrimali]

What does this PR do?

• CI has been failing for Flash CI for the case when a function is converted to ClassFromFunction class. We were not checking if its type is subclass of CustomClassFunctionBase.
• For JIT tracing, the model needs to be attached to the Trainer class, else it will give an error similar to: ImageClassifier is not attached to Trainer.
• Baal integration fails on recent Lightning Trainer versions. on_predict_dataloader, on_train_dataloader, on_test_dataloader were deprecated, and removed in PL 1.7. I'm not very sure if we need them, removing that works?
• For BaaL integration, probabilities need to have a check if it's not empty else torch.cat will fail.

This PR attempts to fix both of the issues. Note that we should document that the best way to JIT trace the model in Flash is to:

# 2. Build the task
model = ImageClassifier(backbone="resnet18", labels=datamodule.labels)

# 3. Create the trainer and finetune the model
trainer = flash.Trainer(max_epochs=3, gpus=torch.cuda.device_count())

# Attach trainer to the model object
model.trainer = trainer
trace = torch.jit.trace(model, torch.randn((3, 196, 196))

Issue: #1413

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[codecov[bot]]

Codecov Report

Merging #1410 (8e03fd7) into master (8737ee8) will decrease coverage by 0.00%. The diff coverage is 85.00%.

@@            Coverage Diff             @@
##           master    #1410      +/-   ##
==========================================
- Coverage   91.77%   91.76%   -0.01%     
==========================================
  Files         286      286              
  Lines       12857    12861       +4     
==========================================
+ Hits        11799    11802       +3     
- Misses       1058     1059       +1     

Flag	Coverage Δ
unittests	91.76% <85.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/utilities/lightning_cli.py	98.16% <ø> (ø)
flash/core/integrations/icevision/adapter.py	94.78% <80.00%> (-1.52%)	:arrow_down:
...ash/image/classification/integrations/baal/data.py	87.96% <100.00%> (ø)
...ash/image/classification/integrations/baal/loop.py	94.81% <100.00%> (-0.12%)	:arrow_down:
flash/image/embedding/model.py	98.48% <100.00%> (ø)
flash/image/embedding/vissl/hooks.py	98.24% <100.00%> (ø)
flash/text/question_answering/model.py	94.55% <0.00%> (+0.68%)	:arrow_up:

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[krshrimali]

About the 5 failures left in the CI:

1. CI testing / pytest (ubuntu-20.04, 3.9, latest, stable, graph) (pull_request): This failure is flaky, and has been fixed in PL. Once there is another release of PL, we should be good.
2. CI testing / pytest (ubuntu-20.04, 3.9, latest, stable, audio) (pull_request): PR https://github.com/Lightning-AI/lightning-flash/pull/1409 is opened to fix it.
3. Lightning-AI.lightning-flash (Special): Horovod versioning error, but I also see /usr/bin/pip not found error. I plan to fix this in a separate PR, doesn't seem to be very urgent as the first step was to support PL 1.7 with Flash.

Thanks to @otaj for his help with a few failures related to PL 1.7, it was very helpful. ❤️ Also @rohitgr7 for a fix! 🔥

cc: @ethanwharris @Borda @carmocca

[krshrimali]

Hi, @ethanwharris - The 5 failures are expected in the CI, only 1 of these is something that I haven't investigated yet. Fixes for others are either already merged in PL, or we have a PR opened in Flash. Could you please take a look again, and help merge if it all looks good? Thanks!