Fix false __len__ claims in Trainer docstrings
Author: albertvillanova
Pull request: yes

<!-- ci-dashboard-badge:start -->
[![CI](https://transformers-ci.lor-e.huggingface.cool/badge/pr?pr=47131)](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=47131)
<!-- ci-dashboard-badge:end -->

# What does this PR do?

This PR removes the false statement that evaluation and test datasets passed to the Trainer must implement the `__len__` method.

### Motivation

The docstrings for evaluate, predict, get_eval_dataloader, and get_test_dataloader all claimed the dataset must implement __len__. This is not true: the evaluation loop guards every length access with has_length(...), the evaluation sampler is None for datasets without a length, and the number of samples falls back to the count observed while iterating. Iterable and streaming datasets are therefore fully supported for evaluation and prediction, and the docstrings misled users into believing otherwise.

### Changes

- Drop the "must implement `__len__`" clause from the `eval_dataset` argument of `get_eval_dataloader`
- Drop the "It must implement `__len__`." sentence from the `test_dataset` argument of `get_test_dataloader`
- Drop the "Datasets must implement the `__len__` method." sentence from the `eval_dataset` argument of `evaluate`
- Drop the "Has to implement the method `__len__`" sentence from `test_dataset` argument of `predict`

## Code Agent Policy

The Transformers repo is currently being overwhelmed by a large number of PRs and issue comments written by
code agents. We are currently bottlenecked by our ability to review and respond to them. As a result, 
**we ask that new users do not submit pure code agent PRs** at this time. 
You may use code agents in drafting or to help you diagnose issues. We'd also ask autonomous "OpenClaw"-like agents
not to open any PRs or issues for the moment.

PRs that appear to be fully agent-written will probably be closed without review, and we may block users who do this
repeatedly or maliciously. 

This is a rapidly-evolving situation that's causing significant shockwaves in the open-source community. As a result, 
this policy is likely to be updated regularly in the near future. For more information, please read [`CONTRIBUTING.md`](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md).

- [x] I confirm that this is not a pure code agent PR.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

CC: @SunMarc

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- text models: @ArthurZucker @Cyrilvallez @vasqu
- vision models: @yonigozlan @molbap
- audio models: @eustlb @ebezzam @vasqu
- multimodal models: @zucchini-nlp
- graph models: @clefourrier

Library:

- generate: @zucchini-nlp (visual-language models) or @Cyrilvallez (all others)
- continuous batching: @remi-or @ArthurZucker @McPatate
- pipelines: @Rocketknight1
- tokenizers: @ArthurZucker and @itazap
- trainer: @SunMarc
- attention: @vasqu @ArthurZucker @CyrilVallez
- model loading (from pretrained, etc): @CyrilVallez
- distributed: @3outeille @ArthurZucker
- CIs: @ydshieh

Integrations:

- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization: @SunMarc
- kernels: @vasqu @drbh
- peft: @BenjaminBossan @githubnemo

Devices/Backends:

- AMD ROCm: @ivarflakstad
- Intel XPU: @IlyasMoutawwakil
- Ascend NPU: @ivarflakstad 

Documentation: @stevhliu

Research projects are not maintained and should be taken as is.

 -->

Comments:
<!-- ci-dashboard-recap:start -->

---

### CI recap

**Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=47131)
**Latest run:** [28855433452](https://github.com/huggingface/transformers/actions/runs/28855433452)
**Result:** `success` | Grafana metrics are not available yet.

<!-- ci-dashboard-recap:end -->

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_47131). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.