Fix token_ids list truncation to specified max_len
# What does this PR do?

Fixed samples processing:
- removed multiple conversions from text_to_ids and ids_to_tokens which causes the difference in the token amounts between these operations
- changed the prompt template processing: tokens inside the prompt template (eg "<extra_token_1") are converted to token_ids and after merging with "input" and "output" token_ids lists 
- due to some tokenizers have no specified space tokens (like " " and "\n") the prompt_template is mandatory (or it will be set by default as "{input} <extra_id_1> {output}"

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add specific line by line info of high level changes in this PR.

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- [+] Bugfix
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# Additional Information
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