Cost is nan When adding guided alignment

[mahmoudaymo]

Bug description

I have trained a model for 5 epochs without guided alignment. Then I trained for 5 epochs more with guided alignment. When training without guided alignment everything went fine. However, when adding the guided alignment (the second 5 epochs) the training cost is nan in every update.

How to reproduce

Describe steps or include command to reproduce the behavior.
I have run this script:

`#!/bin/bash

set -e

exp_dir=path_to_experiment_dir

exp=$exp_dir/basemodel
config=$exp/config.yml

/marian/build/marian -c $config
--valid-log $exp/valid.log
--log $exp/train.log
--model $exp/model.npz
--after 5e

exp=$exp_dir/finetuned
config=$exp/config.yml # This config is similar to the above except I unset --all-caps-every and --english-title-case-every params

/marian/build/marian -c $config
--pretrained-model $pretrained_model_path
--valid-log $exp/valid.log
--log $exp/train.log
--model $exp/model.npz
--after 10e
--guided-alignment /Engines/MAS/ENUSDEDE/alignment/corpus.align
--guided-alignment-cost ce`
marian.logs.txt

Context

• Marian version: 1.12.0
• CMake command: Type the cmake command you used and attach the output of --build-info all
• Log file: Attach your training/decoding logs

Add any other information about the problem here.

[TransperfectAI]

We are experiencing this issue, too, even when training with alignment from the start. Could it be related to the guided-alignment-cost? We used to use mse and then changed to ce when mse was no longer supported. The issue started after that for us.
It also means that to restart training in a directory you need to edit the cost in the model.npz.progress.yml or it throws an error