feat(dataloader): refine implementation of mocked and megatron dataloader

[zigzagcai]

Motivation

1. Fix CI timeout for [Bug] There will be timeout in some cases. #342 (Completed)
2. Refine implementation of megatron and mocked dataloader (Completed)

Modification

• internlm/train/pipeline.py
• internlm/data/*

BC-breaking (Optional)

None

Use cases (Optional)

None

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects.
• CLA has been signed and all committers have signed the CLA in this PR.