docs(APIs): RFC for enhancing paddle.metric.Metric with TorchMetrics-style infrastructure

[PlumBlossomMaid]

Add design document for enhancing paddle.metric.Metric from a minimal ABC to a full-featured base class inspired by TorchMetrics.

This RFC proposes:

• Inheriting paddle.nn.Layer for automatic device management and serialization
• declare() (alias add_state) for declarative state management
• dist_reduce_fn (alias dist_reduce_fx) for distributed reduction
• forward() as unified entry point
• MetricCollection and CompositionalMetric for metric composition
• 40+ new classification and regression metrics with task-based dispatch

Includes three-framework comparison (PaddlePaddle / MindSpore / TorchMetrics) demonstrating the gap analysis and design rationale.

Related PR: https://github.com/PlumBlossomMaid/Paddle/pull/new/feature/optimize-metrics

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