Access all batch outputs at the end of epoch in callback with pytorch lightning
The documentation for the on_train_epoch_end, https://pytorch-lightning.readthedocs.io/en/stable/extensions/callbacks.html#on-train-epoch-end, states:

To access all batch outputs at the end of the epoch, either:

Implement training_epoch_end in the LightningModule and access outputs via the module OR
Cache data across train batch hooks inside the callback implementation to post-process in this hook.


I am trying to use the first alternative with the following LightningModule and Callback setup:
import pytorch_lightning as pl
from pytorch_lightning import Callback

class LightningModule(pl.LightningModule):
    def __init__(self, *args):
        super().__init__()
        self.automatic_optimization = False

    def training_step(self, batch, batch_idx):
        return {'batch': batch}

    def training_epoch_end(self, training_step_outputs):
        # training_step_outputs has all my batches
        return

class MyCallback(Callback):
    def on_train_epoch_end(self, trainer, pl_module):
        # pl_module.batch ???
        return

How do I access the outputs via the pl_module in the callback? What is the recommended way of getting access to training_step_outputs in my callback?

You can store the outputs of each training batch in a state and access it at the end of the training epoch. Here is an example -
from pytorch_lightning import Callback


class MyCallback(Callback):
    def __init__(self):
        super().__init__()
        self.state = []
        
    def on_train_batch_end(self, trainer, pl_module, outputs, batch, batch_idx, unused=0):
        self.state.append(outputs)
        
    def on_train_epoch_end(self, trainer, pl_module):
        # access output using state
        all_outputs = self.state

Hope this helps you! 
