build: Remove dill dependency

[julian-risch]

Related Issues

• Part of Extract dependencies from core list #4417

Proposed Changes:

Remove dill dependency and remove dill from from saving models via torch.save with pickle_module set to dill.

How did you test it?

• Trained a tiny DPR model with the dill module installed: https://colab.research.google.com/drive/1jgjn_4cdbmxK7ZFXeRXTbjxrgqKDQBHD?usp=sharing
• Loaded the trained DPR model here without installing dill: https://colab.research.google.com/drive/1xDpRXh-ArDKLqz4q1PzYyVrWVwefiZfm?usp=sharing

Notes for the reviewer

dill was introduced in FARM in deepset-ai/FARM#271 At that time we used torch.save to not only store the state dicts but also the model itself. As the AdaptiveModel allows defining a loss function as a callable, it was not serializable out of the box and we introduced dill to save it.
In Haystack, we're not storing the model class though and thus I believe dill is not necessary anymore. What remains to be checked is whether model saved with dill can be loaded without. If it's compatible, we can remove dill otherwise we need to keep it.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 5091342081

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 470 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.09%) to 39.619%

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Files with Coverage Reduction	New Missed Lines	%
nodes/file_converter/init.py	1	93.33%
agents/base.py	2	91.25%
init.py	2	83.33%
utils/openai_utils.py	13	75.64%
nodes/answer_generator/openai.py	17	72.22%
nodes/prompt/prompt_template.py	17	90.06%
nodes/retriever/_openai_encoder.py	26	30.68%
nodes/prompt/invocation_layer/open_ai.py	29	30.26%
utils/doc_store.py	30	27.59%
nodes/prompt/prompt_node.py	53	59.34%

Totals
Change from base Build 5055409480:	0.09%
Covered Lines:	8875
Relevant Lines:	22401

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💛 - Coveralls

[bogdankostic]

LGTM