text: Title: Unable to load transformer.wmt18.en-de
  Body: ## 🐛 Bug
  
  Loading transformer.wmt18.en-de` from the hub fails despite working in #1287.
  The checkpoint has to be specified manually (otherwise a different error appears because `model.pt` does not exist in the archive) but that errors as well.
  
  ### To Reproduce
  
  ```python3
  import torch
  torch.hub.load(
      'pytorch/fairseq', 'transformer.wmt18.en-de',
      checkpoint_file='wmt18.model1.pt',
      tokenizer='moses', bpe='subword_nmt'
  )
  ```
  
  Resulting error output:
  
  ```
  Traceback (most recent call last):
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 513, in _apply_overrides_to_config
      OmegaConf.update(cfg, key, value, merge=True)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/omegaconf.py", line 613, in update
      root.__setattr__(last_key, value)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 285, in __setattr__
      raise e
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 282, in __setattr__
      self.__set_impl(key, value)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 266, in __set_impl
      self._set_item_impl(key, value)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/basecontainer.py", line 398, in _set_item_impl
      self._validate_set(key, value)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 143, in _validate_set
      self._validate_set_merge_impl(key, value, is_assign=True)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/dictconfig.py", line 156, in _validate_set_merge_impl
      self._format_and_raise(
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/base.py", line 95, in _format_and_raise
      format_and_raise(
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/_utils.py", line 694, in format_and_raise
      _raise(ex, cause)
    File "/home/vilda/.local/lib/python3.10/site-packages/omegaconf/_utils.py", line 610, in _raise
      raise ex  # set end OC_CAUSE=1 for full backtrace
  omegaconf.errors.ValidationError: child 'checkpoint.save_interval_updates' is not Optional
          full_key: checkpoint.save_interval_updates
          reference_type=CheckpointConfig
          object_type=CheckpointConfig
  
  The above exception was the direct cause of the following exception:
  
  Traceback (most recent call last):
    File "/home/vilda/mt-metric-estimation/./src/retranslate.py", line 100, in <module>
      model = MODELS[args.model](args.direction)
    File "/home/vilda/mt-metric-estimation/./src/retranslate.py", line 88, in <lambda>
      "w18t": lambda direction: FairSeqWrap(config=f"transformer.wmt18.{direction}"),
    File "/home/vilda/mt-metric-estimation/./src/retranslate.py", line 61, in __init__
      torch.hub.load(
    File "/home/vilda/.local/lib/python3.10/site-packages/torch/hub.py", line 540, in load
      model = _load_local(repo_or_dir, model, *args, **kwargs)
    File "/home/vilda/.local/lib/python3.10/site-packages/torch/hub.py", line 569, in _load_local
      model = entry(*args, **kwargs)
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/models/fairseq_model.py", line 267, in from_pretrained
      x = hub_utils.from_pretrained(
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/hub_utils.py", line 82, in from_pretrained
      models, args, task = checkpoint_utils.load_model_ensemble_and_task(
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/checkpoint_utils.py", line 425, in load_model_ensemble_and_task
      state = load_checkpoint_to_cpu(filename, arg_overrides)
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/checkpoint_utils.py", line 343, in load_checkpoint_to_cpu
      state = _upgrade_state_dict(state)
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/checkpoint_utils.py", line 681, in _upgrade_state_dict
      state["cfg"] = convert_namespace_to_omegaconf(state["args"])
    File "/home/vilda/.local/lib/python3.10/site-packages/fairseq/dataclass/utils.py", line 399, in convert_namespace_to_omegaconf
      composed_cfg = compose("config", overrides=overrides, strict=False)
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/experimental/compose.py", line 31, in compose
      cfg = gh.hydra.compose_config(
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/_internal/hydra.py", line 507, in compose_config
      cfg = self.config_loader.load_configuration(
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 151, in load_configuration
      return self._load_configuration(
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 277, in _load_configuration
      ConfigLoaderImpl._apply_overrides_to_config(config_overrides, cfg)
    File "/home/vilda/.local/lib/python3.10/site-packages/hydra/_internal/config_loader_impl.py", line 520, in _apply_overrides_to_config
      raise ConfigCompositionException(
  hydra.errors.ConfigCompositionException: Error merging override checkpoint.save_interval_updates=null
  ```
  
  ### Environment
  
   - fairseq Version: 0.12.2
   - PyTorch Version: 1.12.1+cu116
   - OS: Ubuntu Linux 22.04
   - How you installed fairseq: pip
   - Python version: 3.10.4
   - CUDA/cuDNN version: 11.6
  
  (note that this issue is unrelated to #4680)