_check_and_init_precision: PrecisionPlugin is enabled even _precision_plugin_flag is None

[wohenniubi]

In lightning 1.6.x, for the code in accelerator_connector.py#L645 ~ accelerator_connector.py#L658,
https://github.com/PyTorchLightning/pytorch-lightning/blob/dd475183227644a8d22dca3deb18c99fb0a9b2c4/pytorch_lightning/trainer/connectors/accelerator_connector.py#L645

It's better to enable them only when self._precision_plugin_flag is not None. E.g.

  if self._precision_plugin_flag != None:
      if isinstance(self.accelerator, IPUAccelerator):
           return IPUPrecisionPlugin(self._precision_flag)  # type: ignore
       if isinstance(self.accelerator, HPUAccelerator):
           return HPUPrecisionPlugin(self._precision_flag)  # type: ignore
       if isinstance(self.accelerator, TPUAccelerator):
           if self._precision_flag == 32:
               return TPUPrecisionPlugin()
           elif self._precision_flag in (16, "bf16"):
               if self._precision_flag == 16:
                   rank_zero_warn(
                       "You passed `Trainer(accelerator='tpu', precision=16)` but AMP"
                       " is not supported with TPUs. Using `precision='bf16'` instead."
                   )
               return TPUBf16PrecisionPlugin()

[akihironitta]

Hi @wohenniubi, can I ask what makes you think so? Are you seeing any misbehaviour?

---

If a user gives a PrecisionPlugin (self._precision_plugin_flag), the accelerator connector checks if the provided plugin is a valid one in:
https://github.com/PyTorchLightning/pytorch-lightning/blob/dd475183227644a8d22dca3deb18c99fb0a9b2c4/pytorch_lightning/trainer/connectors/accelerator_connector.py#L283-L314
and in this method:
https://github.com/PyTorchLightning/pytorch-lightning/blob/dd475183227644a8d22dca3deb18c99fb0a9b2c4/pytorch_lightning/trainer/connectors/accelerator_connector.py#L702

So, if the provided PrecisionPlugin is valid, the following line will return the plugin as is,
https://github.com/PyTorchLightning/pytorch-lightning/blob/dd475183227644a8d22dca3deb18c99fb0a9b2c4/pytorch_lightning/trainer/connectors/accelerator_connector.py#L642-L643

but if not, it'll be guaranteed that self._precision_plugin_flag is None there, and thus we don't need to check it.