TypeError: argument of type 'PosixPath' is not iterable (in modeling_utils.py)
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....):BERT

Language I am using the model on (English, Chinese....):English

The problem arise when using:
* [x] the official example scripts: (give details)
* [ ] my own modified scripts: (give details)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details)

## To Reproduce
TypeError: argument of type 'PosixPath' is not iterable 

Steps to reproduce the behavior:

1.install transformers by pip
2.make test function with model=Bert.....    line, put your own values as arguements


<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->
```
Traceback (most recent call last):
  File "run_bert.py", line 226, in <module>
    main()
  File "run_bert.py", line 219, in main
    run_train(args)
  File "run_bert.py", line 70, in run_train
    model = BertForMultiLable.from_pretrained(config['bert_model_dir'], num_labels=len(label_list))
  File "/home/aditya/anaconda3/envs/RD/lib/python3.7/site-packages/transformers/modeling_utils.py", line 321, in from_pretrained
    if "albert" in pretrained_model_name_or_path and "v2" in pretrained_model_name_or_path:
TypeError: argument of type 'PosixPath' is not iterable

```
run_train():
```

def run_train(args):
    # --------- data
    processor = BertProcessor(vocab_path=config['bert_vocab_path'], do_lower_case=args.do_lower_case)
    label_list = processor.get_labels()
    label2id = {label: i for i, label in enumerate(label_list)}
    id2label = {i: label for i, label in enumerate(label_list)}

    train_data = processor.get_train(config['data_dir'] / f"{args.data_name}.train.pkl")
    train_examples = processor.create_examples(lines=train_data,
                                               example_type='train',
                                               cached_examples_file=config[
                                                    'data_dir'] / f"cached_train_examples_{args.arch}")
    train_features = processor.create_features(examples=train_examples,
                                               max_seq_len=args.train_max_seq_len,
                                               cached_features_file=config[
                                                    'data_dir'] / "cached_train_features_{}_{}".format(
                                                   args.train_max_seq_len, args.arch
                                               ))
    train_dataset = processor.create_dataset(train_features, is_sorted=args.sorted)
    if args.sorted:
        train_sampler = SequentialSampler(train_dataset)
    else:
        train_sampler = RandomSampler(train_dataset)
    train_dataloader = DataLoader(train_dataset, sampler=train_sampler, batch_size=args.train_batch_size)

    valid_data = processor.get_dev(config['data_dir'] / f"{args.data_name}.valid.pkl")
    valid_examples = processor.create_examples(lines=valid_data,
                                               example_type='valid',
                                               cached_examples_file=config[
                                                                        'data_dir'] / f"cached_valid_examples_{args.arch}")

    valid_features = processor.create_features(examples=valid_examples,
                                               max_seq_len=args.eval_max_seq_len,
                                               cached_features_file=config[
                                                                        'data_dir'] / "cached_valid_features_{}_{}".format(
                                                   args.eval_max_seq_len, args.arch
                                               ))
    valid_dataset = processor.create_dataset(valid_features)
    valid_sampler = SequentialSampler(valid_dataset)
    valid_dataloader = DataLoader(valid_dataset, sampler=valid_sampler, batch_size=args.eval_batch_size)

    # ------- model
    logger.info("initializing model")
    if args.resume_path:
        args.resume_path = Path(args.resume_path)
        model = BertForMultiLable.from_pretrained(args.resume_path, num_labels=len(label_list))
    else:
        model = BertForMultiLable.from_pretrained(config['bert_model_dir'], num_labels=len(label_list))
    t_total = int(len(train_dataloader) / args.gradient_accumulation_steps * args.epochs)

    param_optimizer = list(model.named_parameters())
    no_decay = ['bias', 'LayerNorm.weight']
    optimizer_grouped_parameters = [
        {'params': [p for n, p in param_optimizer if not any(nd in n for nd in no_decay)],'weight_decay': args.weight_decay},
        {'params': [p for n, p in param_optimizer if any(nd in n for nd in no_decay)], 'weight_decay': 0.0}
    ]
    warmup_steps = int(t_total * args.warmup_proportion)
    optimizer = AdamW(optimizer_grouped_parameters, lr=args.learning_rate, eps=args.adam_epsilon)
    lr_scheduler = get_linear_schedule_with_warmup(optimizer, num_warmup_steps=warmup_steps, num_training_steps=t_total)

    if args.fp16:
        try:
            from apex import amp
        except ImportError:
            raise ImportError("Please install apex from https://www.github.com/nvidia/apex to use fp16 training.")
        model, optimizer = amp.initialize(model, optimizer, opt_level=args.fp16_opt_level)

    # ---- callbacks
    logger.info("initializing callbacks")
    train_monitor = TrainingMonitor(file_dir=config['figure_dir'], arch=args.arch)
    model_checkpoint = ModelCheckpoint(checkpoint_dir=config['checkpoint_dir'],mode=args.mode,
                                       monitor=args.monitor,arch=args.arch,
                                       save_best_only=args.save_best)

    # **************************** training model ***********************
    logger.info("***** Running training *****")
    logger.info("  Num examples = %d", len(train_examples))
    logger.info("  Num Epochs = %d", args.epochs)
    logger.info("  Total train batch size (w. parallel, distributed & accumulation) = %d",
                args.train_batch_size * args.gradient_accumulation_steps * (
                    torch.distributed.get_world_size() if args.local_rank != -1 else 1))
    logger.info("  Gradient Accumulation steps = %d", args.gradient_accumulation_steps)
    logger.info("  Total optimization steps = %d", t_total)

    trainer = Trainer(n_gpu=args.n_gpu,
                      model=model,
                      epochs=args.epochs,
                      logger=logger,
                      criterion=BCEWithLogLoss(),
                      optimizer=optimizer,
                      lr_scheduler=lr_scheduler,
                      early_stopping=None,
                      training_monitor=train_monitor,
                      fp16=args.fp16,
                      resume_path=args.resume_path,
                      grad_clip=args.grad_clip,
                      model_checkpoint=model_checkpoint,
                      gradient_accumulation_steps=args.gradient_accumulation_steps,
                      batch_metrics=[AccuracyThresh(thresh=0.5)],
                      epoch_metrics=[AUC(average='micro', task_type='binary'),
                                     MultiLabelReport(id2label=id2label)])
    trainer.train(train_data=train_dataloader, valid_data=valid_dataloader, seed=args.seed)



```


## Expected behavior

<!-- A clear and concise description of what you expected to happen. -->

## Environment

* OS:Ubuntu 18.04
* Python version:3.6
* PyTorch version:
* PyTorch Transformers version (or branch):2.2.1
* Using GPU ?No
* Distributed of parallel setup ?Non Distrubuted
* Any other relevant information:

## Additional context

<!-- Add any other context about the problem here. -->

Solved it by typecasting posixpath to string