Fix LightningModule.{un,}toggle_model when only 1 optimizer is used

[CAIQT]

What does this PR do?

Fixes #12086.

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[carmocca]

Good catch! Can you add a simple test calling toggle and untoggle in the file tests/core/test_lightning_module.py?

You can mock the Trainer.

[carmocca]

Good catch! Can you add a simple test calling toggle and untoggle in the file tests/core/test_lightning_module.py?

You can mock the Trainer.

[carmocca]

Good catch! Can you add a simple test calling toggle and untoggle in the file tests/core/test_lightning_module.py?

You can mock the Trainer.

[tchaton]

LGTM !