Add support for iterable datasets when val_check_interval=1.0

[ethanwharris]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• not yet, see below
• Did you write any new necessary tests?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Fixes #1252

• had to make a method with will look ahead one batch - which may be a memory issue (but since we usually use multi-threaded data loading anyway should be fine) - can't see any other way to do it
• docs will need to be updated after Update docs iterable datasets #1281

PR review

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If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

Merging #1283 into master will increase coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #1283    +/-   ##
=======================================
+ Coverage      92%     92%   +<1%     
=======================================
  Files          61      61            
  Lines        3153    3164    +11     
=======================================
+ Hits         2886    2897    +11     
  Misses        267     267

[Borda]

the multiline bool expression is quite hard to read...