Add `examples/language_modeling/run_mlm_no_trainer.py`
# What does this PR do?

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This PR adds an example of finetuning a Masked Language Model (without using `Trainer`) to show the functionalities of the new accelerate library.



## Who can review?

@sgugger 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
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Thanks again!<|||||>How to distributed training?~~~ In no trainer mlm<|||||>The same way as any other scripts: `python -m torch.distributed.launch --nproc_per_node xxx run_mlm_no_trainer.py --script_args`.<|||||>hello@sgugger , when I used multi-gpu, I got this error message:
(basic_dl) root@PM00011093:/data/zhaoyichen/workplace/transformers-master/examples# python -m torch.distributed.launch \
>     --nproc_per_node 2 pytorch/language-modeling/run_mlm_no_trainer.py \
>     --dataset_name wikitext \
>     --dataset_config_name wikitext-2-raw-v1 \
>     --model_name_or_path roberta-base \
>     --output_dir /tmp/test-mlm
*****************************************
Setting OMP_NUM_THREADS environment variable for each process to be 1 in default, to avoid your system being overloaded, please further tune the variable for optimal performance in your application as needed. 
*****************************************
usage: run_mlm_no_trainer.py [-h] [--dataset_name DATASET_NAME]
                             [--dataset_config_name DATASET_CONFIG_NAME]
                             [--train_file TRAIN_FILE]
                             [--validation_file VALIDATION_FILE]
                             [--validation_split_percentage VALIDATION_SPLIT_PERCENTAGE]
                             [--pad_to_max_length] --model_name_or_path
                             MODEL_NAME_OR_PATH [--config_name CONFIG_NAME]
                             [--tokenizer_name TOKENIZER_NAME]
                             [--use_slow_tokenizer]
                             [--per_device_train_batch_size PER_DEVICE_TRAIN_BATCH_SIZE]
                             [--per_device_eval_batch_size PER_DEVICE_EVAL_BATCH_SIZE]
                             [--learning_rate LEARNING_RATE]
                             [--weight_decay WEIGHT_DECAY]
                             [--num_train_epochs NUM_TRAIN_EPOCHS]
                             [--max_train_steps MAX_TRAIN_STEPS]
                             [--gradient_accumulation_steps GRADIENT_ACCUMULATION_STEPS]
                             [--lr_scheduler_type {linear,cosine,cosine_with_restarts,polynomial,constant,constant_with_warmup}]
                             [--num_warmup_steps NUM_WARMUP_STEPS]
                             [--output_dir OUTPUT_DIR] [--seed SEED]
                             [--model_type {clip,bigbird_pegasus,deit,luke,gpt_neo,big_bird,speech_to_text,vit,wav2vec2,m2m_100,convbert,led,blenderbot-small,retribert,mt5,t5,pegasus,marian,mbart,blenderbot,distilbert,albert,camembert,xlm-roberta,bart,longformer,roberta,layoutlm,squeezebert,bert,openai-gpt,gpt2,megatron-bert,mobilebert,transfo-xl,xlnet,flaubert,fsmt,xlm,ctrl,electra,reformer,funnel,lxmert,bert-generation,deberta,deberta-v2,dpr,xlm-prophetnet,prophetnet,mpnet,tapas,ibert}]
                             [--max_seq_length MAX_SEQ_LENGTH]
                             [--line_by_line LINE_BY_LINE]
                             [--preprocessing_num_workers PREPROCESSING_NUM_WORKERS]
                             [--overwrite_cache OVERWRITE_CACHE]
                             [--mlm_probability MLM_PROBABILITY]
run_mlm_no_trainer.py: error: unrecognized arguments: --local_rank=0
usage: run_mlm_no_trainer.py [-h] [--dataset_name DATASET_NAME]
                             [--dataset_config_name DATASET_CONFIG_NAME]
                             [--train_file TRAIN_FILE]
                             [--validation_file VALIDATION_FILE]
                             [--validation_split_percentage VALIDATION_SPLIT_PERCENTAGE]
                             [--pad_to_max_length] --model_name_or_path
                             MODEL_NAME_OR_PATH [--config_name CONFIG_NAME]
                             [--tokenizer_name TOKENIZER_NAME]
                             [--use_slow_tokenizer]
                             [--per_device_train_batch_size PER_DEVICE_TRAIN_BATCH_SIZE]
                             [--per_device_eval_batch_size PER_DEVICE_EVAL_BATCH_SIZE]
                             [--learning_rate LEARNING_RATE]
                             [--weight_decay WEIGHT_DECAY]
                             [--num_train_epochs NUM_TRAIN_EPOCHS]
                             [--max_train_steps MAX_TRAIN_STEPS]
                             [--gradient_accumulation_steps GRADIENT_ACCUMULATION_STEPS]
                             [--lr_scheduler_type {linear,cosine,cosine_with_restarts,polynomial,constant,constant_with_warmup}]
                             [--num_warmup_steps NUM_WARMUP_STEPS]
                             [--output_dir OUTPUT_DIR] [--seed SEED]
                             [--model_type {clip,bigbird_pegasus,deit,luke,gpt_neo,big_bird,speech_to_text,vit,wav2vec2,m2m_100,convbert,led,blenderbot-small,retribert,mt5,t5,pegasus,marian,mbart,blenderbot,distilbert,albert,camembert,xlm-roberta,bart,longformer,roberta,layoutlm,squeezebert,bert,openai-gpt,gpt2,megatron-bert,mobilebert,transfo-xl,xlnet,flaubert,fsmt,xlm,ctrl,electra,reformer,funnel,lxmert,bert-generation,deberta,deberta-v2,dpr,xlm-prophetnet,prophetnet,mpnet,tapas,ibert}]
                             [--max_seq_length MAX_SEQ_LENGTH]
                             [--line_by_line LINE_BY_LINE]
                             [--preprocessing_num_workers PREPROCESSING_NUM_WORKERS]
                             [--overwrite_cache OVERWRITE_CACHE]
                             [--mlm_probability MLM_PROBABILITY]
run_mlm_no_trainer.py: error: unrecognized arguments: --local_rank=1
Traceback (most recent call last):
  File "/root/anaconda3/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/root/anaconda3/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/data/zhaoyichen/pyvenv/basic_dl/lib/python3.6/site-packages/torch/distributed/launch.py", line 261, in <module>
    main()
  File "/data/zhaoyichen/pyvenv/basic_dl/lib/python3.6/site-packages/torch/distributed/launch.py", line 257, in main
    cmd=cmd)
subprocess.CalledProcessError: Command '['/data/zhaoyichen/pyvenv/basic_dl/bin/python', '-u', 'pytorch/language-modeling/run_mlm_no_trainer.py', '--local_rank=1', '--dataset_name', 'wikitext', '--dataset_config_name', 'wikitext-2-raw-v1', '--model_name_or_path', 'roberta-base', '--output_dir', '/tmp/test-mlm']' returned non-zero exit status 2.

<|||||>You need to launch it with `--use_env` when using the PyTorch launcher (or use `accelerate launch`).