Better error message when inconsistent files are specified in read_from for energies and forces

[SanggyuChong]

Hello,

Thanks a lot for all your efforts in mtt development. Nice to be on the user's end!

I committed a mistake in model training today, of providing different file names for the energy and forces to read_from:

training_set:
  systems:
    read_from: VALID.xyz
    length_unit: angstrom
  targets:
    energy:
      key: "DFT_energy"
      unit: "eV" # very important to run simulations
      forces:
        read_from: TRAIN.xyz
        key: "DFT_forces"

Doing something like this currently gives the following error thread:

Traceback (most recent call last):
 File "/home/chong/mtt_venv/lib/python3.11/site-packages/metatrain/cli/train.py", line 400, in train_model
  trainer.train(
 File "/home/chong/mtt_venv/lib/python3.11/site-packages/metatrain/experimental/soap_bpnn/trainer.py", line 336, in train
  targets = remove_additive(
       ^^^^^^^^^^^^^^^^
 File "/home/chong/mtt_venv/lib/python3.11/site-packages/metatrain/utils/additive/remove.py", line 66, in remove_additive
  metatensor.torch.TensorBlock(
RuntimeError: invalid parameter: data and labels don't match: the array shape along axis 0 is 96 but we have 160 sample labels

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/home/chong/mtt_venv/lib/python3.11/site-packages/metatrain/__main__.py", line 107, in main
  train_model(**args.__dict__)
 File "/home/chong/mtt_venv/lib/python3.11/site-packages/metatrain/cli/train.py", line 409, in train_model
  raise ArchitectureError(e)
metatrain.utils.errors.ArchitectureError: invalid parameter: data and labels don't match: the array shape along axis 0 is 96 but we have 160 sample labels

The error above most likely originates from an architecture.

If you think this is a bug, please contact its maintainer (see the architecture's documentation) and include the full traceback error.log.

While the user is ultimately responsible for ensuring that correct file names are provided, @Luthaf has asked me to raise this issue to discuss a way in which one can catch these mistakes with a more straightforward error message.

Thanks a lot!

Best,
Raymond