Allow naming matrix stage expansions

[Kyliroco]

Summary

When a stage uses the matrix fan-out, DVC currently auto-generates suffixes such as stage@set0_set1. I would like to let users supply an optional name template so each matrix entry can be named in a predictable way.

Motivation

• Large pipelines combine many datasets and models. Auto-generated suffixes are opaque, which makes dvc stage list, dvc repro, and log monitoring harder to follow.
• My automation relies on stable stage identifiers that reflect dataset/model keys; today I have to reverse-map dataset0_model0 back to the real combinations with dvc stage list. Allowing for exemple in name"${item.model.key}_${item.dataset.key}" would remove that indirection.

Proposed change

• Accept an optional name field inside matrix stage definitions . The field is validated to ensure the resolved value is non-empty, doesn’t contain the @ separator, and stays unique across the fan-out. ​ link to PR

Example

stages:
  inference:
    matrix:
      dataset: ${datasets_list}
      model: ${models_list}
    name: "${item.model.key}_${item.dataset.key}"
    cmd: >
      inference

dvc stage list would then show entries such as inference@model_alpha_dataset_a instead of inference@dataset0_model0.