feat(dataloader): Implement megatron dataloader and mocked dataloader

[zigzagcai]

This PR's main functionality is okay and runnable, but still need some refinement.

Motivation

• Support megatron dataloader type: When users want to use InternEvo framework to train over megatron tokenized datasets.

• Support mocked dataloader type: When users want to conduct precision alignment experiment to ensure that the loaded data is completely consistent.

Modification

internlm/data/megatron/*
internlm/data/mocked/*

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.

[sunpengsdu]

LGTM