Fine-tuning the 2.7B Blender model raises an attribute error

[NeonAndrii]

Bug description
Attribute error ('TransformerGeneratorAgent' object has no attribute '_fake_forward_pass') raised while fine-tuning the 2.7B model following the tutorial steps.

Reproduction steps
Run the fine-tuning command from the tutorial
parlai train_model -t blended_skill_talk,wizard_of_wikipedia,convai2:normalized,empathetic_dialogues --multitask-weights 1,3,3,3 -veps 0.25 --attention-dropout 0.0 --batchsize 128 --model transformer/generator --embedding-size 2560 --ffn-size 10240 --variant prelayernorm --n-heads 32 --n-positions 128 --n-encoder-layers 2 --n-decoder-layers 24 --history-add-global-end-token end --delimiter ' ' --dict-tokenizer bytelevelbpe --dropout 0.1 --fp16 True --init-model zoo:blender/reddit_3B/model --dict-file zoo:blender/reddit_3B/model.dict --label-truncate 128 --log_every_n_secs 10 -lr 7e-06 --lr-scheduler reduceonplateau --lr-scheduler-patience 3 --optimizer adam --relu-dropout 0.0 --activation gelu --model-parallel true --save-after-valid True --text-truncate 128 --truncate 128 --warmup_updates 100 --fp16-impl mem_efficient --update-freq 2 --gradient-clip 0.1 --skip-generation True -vp 10 -vmt ppl -vmm min --model-file /tmp/test_train_27B

Expected behavior
The process proceeds to fine-tuning the initial 2.7B Blender model on blended_skill_talk, wizard_of_wikipedia, convai2 and empathetic_dialogues tasks.

Logs
Traceback (most recent call last):
File "/home/ubuntu/anaconda3/bin/parlai", line 11, in
load_entry_point('parlai', 'console_scripts', 'parlai')()
File "/home/ubuntu/ParlAI/parlai/main.py", line 14, in main
superscript_main()
File "/home/ubuntu/ParlAI/parlai/core/script.py", line 325, in superscript_main
return SCRIPT_REGISTRY[cmd].klass._run_from_parser_and_opt(opt, parser)
File "/home/ubuntu/ParlAI/parlai/core/script.py", line 108, in _run_from_parser_and_opt
return script.run()
File "/home/ubuntu/ParlAI/parlai/scripts/train_model.py", line 936, in run
return self.train_loop.train()
File "/home/ubuntu/ParlAI/parlai/scripts/train_model.py", line 900, in train
for _train_log in self.train_steps():
File "/home/ubuntu/ParlAI/parlai/scripts/train_model.py", line 803, in train_steps
world.parley()
File "/home/ubuntu/ParlAI/parlai/core/worlds.py", line 865, in parley
batch_act = self.batch_act(agent_idx, batch_observations[agent_idx])
File "/home/ubuntu/ParlAI/parlai/core/worlds.py", line 833, in batch_act
batch_actions = a.batch_act(batch_observation)
File "/home/ubuntu/ParlAI/parlai/core/torch_agent.py", line 2214, in batch_act
output = self.train_step(batch)
File "/home/ubuntu/ParlAI/parlai/core/torch_generator_agent.py", line 753, in train_step
self._fake_forward_pass()
AttributeError: 'TransformerGeneratorAgent' object has no attribute '_fake_forward_pass'

[stephenroller]

Appreciate the speedy report. PR out for fix.