How to test a model before fine-tuning in Pytorch Lightning?
Doing things on Google Colab.

transformers: 4.10.2
pytorch-lightning: 1.2.7

import torch
from torch.utils.data import DataLoader
from transformers import BertJapaneseTokenizer, BertForSequenceClassification
import pytorch_lightning as pl

dataset_for_loader = [
    {'data':torch.tensor([0,1]), 'labels':torch.tensor(0)},
    {'data':torch.tensor([2,3]), 'labels':torch.tensor(1)},
    {'data':torch.tensor([4,5]), 'labels':torch.tensor(2)},
    {'data':torch.tensor([6,7]), 'labels':torch.tensor(3)},
]
loader = DataLoader(dataset_for_loader, batch_size=2)

for idx, batch in enumerate(loader):
    print(f'# batch {idx}')
    print(batch)

category_list = [
    'dokujo-tsushin',
    'it-life-hack',
    'kaden-channel',
    'livedoor-homme',
    'movie-enter',
    'peachy',
    'smax',
    'sports-watch',
    'topic-news'
]

tokenizer = BertJapaneseTokenizer.from_pretrained(MODEL_NAME)

max_length = 128
dataset_for_loader = []
for label, category in enumerate(tqdm(category_list)):
    # file ./text has lots of articles, categorized by category
    # and they are just plain texts, whose content begins from forth line
    for file in glob.glob(f'./text/{category}/{category}*'):
        lines = open(file).read().splitlines()
        text = '\n'.join(lines[3:])
        encoding = tokenizer(
            text,
            max_length=max_length, 
            padding='max_length',
            truncation=True
        )
        encoding['labels'] = label
        encoding = { k: torch.tensor(v) for k, v in encoding.items() }
        dataset_for_loader.append(encoding)

SEED=lambda:0.0

# random.shuffle(dataset_for_loader) # ランダムにシャッフル
random.shuffle(dataset_for_loader,SEED)
n = len(dataset_for_loader)
n_train = int(0.6*n)
n_val = int(0.2*n)
dataset_train = dataset_for_loader[:n_train]
dataset_val = dataset_for_loader[n_train:n_train+n_val]
dataset_test = dataset_for_loader[n_train+n_val:]

dataloader_train = DataLoader(
    dataset_train, batch_size=32, shuffle=True    
) 
dataloader_val = DataLoader(dataset_val, batch_size=256)
dataloader_test = DataLoader(dataset_test, batch_size=256)

class BertForSequenceClassification_pl(pl.LightningModule):
    def __init__(self, model_name, num_labels, lr):
        super().__init__()
        self.save_hyperparameters()
        self.bert_sc = BertForSequenceClassification.from_pretrained(
            model_name,
            num_labels=num_labels
        )

    def training_step(self, batch, batch_idx):
        output = self.bert_sc(**batch)
        loss = output.loss
        self.log('train_loss', loss)
        return loss

    def validation_step(self, batch, batch_idx):
        output = self.bert_sc(**batch)
        val_loss = output.loss
        self.log('val_loss', val_loss)

    def test_step(self, batch, batch_idx):
        labels = batch.pop('labels')
        output = self.bert_sc(**batch)
        labels_predicted = output.logits.argmax(-1)
        num_correct = ( labels_predicted == labels ).sum().item()
        accuracy = num_correct/labels.size(0)
        self.log('accuracy', accuracy)

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters(), lr=self.hparams.lr)

checkpoint = pl.callbacks.ModelCheckpoint(
    monitor='val_loss',
    mode='min',
    save_top_k=1,
    save_weights_only=True,
    dirpath='model/',
)
trainer = pl.Trainer(
    gpus=1,
    max_epochs=10,
    callbacks = [checkpoint]
)

model = BertForSequenceClassification_pl(
    MODEL_NAME, num_labels=9, lr=1e-5
)

### (a) ###

# I think this is where I am doing fine-tuning
trainer.fit(model, dataloader_train, dataloader_val)

# this is to score after fine-tuning
test = trainer.test(test_dataloaders=dataloader_test)
print(f'Accuracy: {test[0][&quot;accuracy&quot;]:.2f}')

But I am not really sure how to do a test before fine-tuning, in order to compare two models before and after fine-tuning, in order to show how effective fine-tuning is.
Inserting the following two lines to ### (a) ###:
test = trainer.test(test_dataloaders=dataloader_test)
print(f'Accuracy: {test[0][&quot;accuracy&quot;]:.2f}')

I got this result:
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
&lt;ipython-input-13-c8b2c67f2d5c&gt; in &lt;module&gt;()
      9 
     10 # 6-19
---&gt; 11 test = trainer.test(test_dataloaders=dataloader_test)
     12 print(f'Accuracy: {test[0][&quot;accuracy&quot;]:.2f}')
     13 

/usr/local/lib/python3.7/dist-packages/pytorch_lightning/trainer/trainer.py in test(self, model, test_dataloaders, ckpt_path, verbose, datamodule)
    896         self.verbose_test = verbose
    897 
--&gt; 898         self._set_running_stage(RunningStage.TESTING, model or self.lightning_module)
    899 
    900         # If you supply a datamodule you can't supply train_dataloader or val_dataloaders

/usr/local/lib/python3.7/dist-packages/pytorch_lightning/trainer/trainer.py in _set_running_stage(self, stage, model_ref)
    563         the trainer and the model
    564         &quot;&quot;&quot;
--&gt; 565         model_ref.running_stage = stage
    566         self._running_stage = stage
    567 

AttributeError: 'NoneType' object has no attribute 'running_stage'

I noticed that Trainer.fit() can take None as arguments other than model, so I tried this:
trainer.fit(model)
test=trainer.test(test_dataloaders=dataloader_test)
print(f'Accuracy: {test[0][&quot;accuracy&quot;]:.2f}')

The result:
MisconfigurationException: No `train_dataloader()` method defined. Lightning `Trainer` expects as minimum a `training_step()`, `train_dataloader()` and `configure_optimizers()` to be defined.

Thanks.

The Trainer needs to call its .fit() in order to set up a lot of things and then only you can do .test() or other methods.
You are right about putting a .fit() just before .test() but the fit call needs to a valid one. You have to feed a dataloader/datamodule to it. But since you don't want to do a training/validation in this fit call, just pass limit_[train/val]_batches=0 while Trainer construction.
trainer = Trainer(gpus=..., ..., limit_train_batches=0, limit_val_batches=0)
trainer.fit(model, dataloader_train, dataloader_val)
trainer.test(model, dataloader_test) # without fine-tuning

The fit call here will just set things up for you and skip training/validation. And then the testing follows. Next time run the same code but without the limit_[train/val]_batches, this will do the pretraining for you
trainer = Trainer(gpus=..., ...)
trainer.fit(model, dataloader_train, dataloader_val)
trainer.test(model, dataloader_test) # with fine-tuning


Clarifying a bit about .fit() taking None for all but model: Its not quite true - you must provide either a DataLoader or a DataModule.
