[1.6] Make IterableDataset DataLoader.__len__ warning clearer

[ssnl]

No description provided.

[dr-ci]

💊 CI failures summary and remediations

As of commit a8d8116 (more details on the Dr. CI page):

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💚 💚 Looks good so far! There are no failures yet. 💚 💚

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1 failure confirmed as flaky and can be ignored:

• caffe2_onnx_ort2_py3_6_clang7_ubuntu16_04_test

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This comment has been revised 2 times.

[ssnl]

@zou3519 merge? :)

[zou3519]

@ssnl still waiting for #41175 to land (I hit the land button for that). Once that lands we request to merge this into the release branch

EDIT: #41175 landed, I've submitted a request to merge to the release tracker