Logging with Fabric using steps

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### Description & Motivation

Logging using Fabric does not consider any steps during training, unlike when using the Lightning Trainer. A LightningModule calling self.log simply passes the logged dictionary and nothing else to the Fabric logging code when using Fabric but when using the Trainer it is handled by grouping/frequency adjustments (such as aggregating during multi-gpu training or logging every X steps [default 50]).

### Pitch

An option to enable similar logging in Fabric as the Lightning Trainer. This could be off by default but could track steps that are submitted with fabric hooks/calls, such as:

`fabric.call('on_train_step')`

This would allow for logged values to be aggregated during the same step, which makes logs more readable.

### Alternatives

_No response_

### Additional context

_No response_

cc @lantiga @borda @justusschock

Start by tracing Fabric's logging path and comparing it with the Lightning Trainer's handling of self.log, including grouping and frequency adjustments. Check how fabric.call('on_train_step') exposes step information, then define completion as optional step-aware aggregation that preserves current behavior when disabled and produces readable logs for values from the same step.

Written by the indexing model from the issue text.