Fix 'ValueError' in 'ct2-opennmt-py-converter' for Unsupported '--self_attn_type'

[chingizof]

Description:

This pull request resolves a ValueError when converting OpenNMT models to CTranslate2 using ct2-opennmt-py-converter. The issue was due to unsupported --self_attn_type scaled-dot-flash, with only scaled-dot being supported.

Solution:
Implemented a fix based on a suggestion from a discussion by vince62s on the OpenNMT forum, which successfully addresses the conversion issue.

Testing:
Verified the fix by converting models that previously triggered the error, ensuring the process now completes without issues.

Reference:

Solution inspired by a post on OpenNMT Forum.
This update should help users facing similar conversion problems.

[vince62s]

Hi, my message on the forum was a hack not really a fix.
In fact a better fix is to replace (when applicable) "scaled-dot-flash" by "scaled-dot" because in onmt-py we added "scaled-dot-flash" to force decoding using flash attention but it has no real usage in ct2.

[vince62s]

closing in favor of #1687