ELMo fine-tuning works incorrectly for different `save_path` and `load_path`

Created by: dilyararimovna

Fine-tuning of ELMo embedding model works incorrectly (does not train at all) if all save_paths in config are changed to something different from corresponding load_paths. For example, the config below does not work properly:

{
  "dataset_reader": {
    "class_name": "file_paths_reader",
    "data_path": "{DOWNLOADS_PATH}/elmo-lm-ready4fine-example-data/data/",
    "train": "train/*",
    "valid": "heldout/*"
  },
  "dataset_iterator": {
    "class_name": "elmo_file_paths_iterator",
    "seed": 31415,
    "unroll_steps": 20,
    "max_word_length": 50,
    "n_gpus": 1,
    "shuffle": false,
    "bos":"<S>",
    "eos":"</S>",
    "save_path": "{MODELS_PATH}/fine-tuned/vocab.txt",
    "load_path":"{MODELS_PATH}/elmo-lm-ready4fine-tuning-ru-twitter/vocab.txt"
  },
  "chainer": {
    "in": [
      "x_char_ids"
    ],
    "in_y": [
      "y_token_ids"
    ],
    "pipe": [
      {
        "class_name": "elmo_model",
        "options_json_path": "{MODELS_PATH}/elmo-lm-ready4fine-tuning-ru-twitter/options.json",
        "unroll_steps": 20,
        "batch_size": 128,
        "save_path": "{MODELS_PATH}/fine-tuned/saves/model",
        "load_path": "{MODELS_PATH}/elmo-lm-ready4fine-tuning-ru-twitter/saves/model",
        "in": ["x_char_ids", "y_token_ids"],
        "in_y": [],
        "n_gpus": 1,
        "out": ["loss"]
      }
    ],
   "out": [
    "x_char_ids",
    "y_token_ids"
  ]
  },
  "train": {
    "epochs": 20,
    "batch_size": 128,
    "log_every_n_epochs": 1,
    "val_every_n_epochs": 1,
    "validation_patience": 20,
    "metric_optimization": "minimize",
    "metrics": [
      {
        "name": "elmo_loss2ppl",
        "inputs": ["loss"]
      }
    ],
    "tensorboard_log_dir": "{MODELS_PATH}/fine-tuned/logs"
  },
  "metadata": {
    "variables": {
      "ROOT_PATH": "~/.deeppavlov",
      "DOWNLOADS_PATH": "{ROOT_PATH}/downloads",
      "MODELS_PATH": "{ROOT_PATH}/models"
    },
    "requirements": [
      "{DEEPPAVLOV_PATH}/requirements/tf.txt",
      "{DEEPPAVLOV_PATH}/requirements/tf-hub.txt"
    ],
    "download": [
      {
        "url": "http://files.deeppavlov.ai/deeppavlov_data/elmo-lm-ready4fine-example-data.tar.gz",
        "subdir": "{DOWNLOADS_PATH}/"
      },
      {
        "url": "http://files.deeppavlov.ai/deeppavlov_data/elmo-lm-ready4fine-tuning-ru-twitter.tar.gz",
        "subdir": "{MODELS_PATH}/"
      }
    ]
  }
}