perf: optimize the performance of circular buffer

[iansseijelly]

Description

This PR addresses issue #4274.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

Training throughput before and after the patch running training on task [Isaac-Velocity-Flat-Spot-v0].

## Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

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Greptile Summary

Optimized circular buffer performance by eliminating GPU-CPU synchronization in the hot path, addressing the performance issue identified in #4274.

Key optimizations:

• Cached max_length as an integer to avoid repeated .item() calls that trigger GPU sync
• Added _all_initialized flag to skip first-push checks after warmup, avoiding torch.any().item() calls
• Removed unnecessary .clone() in DelayBuffer.compute() return path
• Changed to in-place assignment ([:]) in DelayedPDActuator to avoid tensor reallocation

Performance impact:
According to the PR description, profiling showed excessive time spent in aten::item (89% CPU time) and torch.any checks. These optimizations eliminate both bottlenecks in steady-state operation after warmup.

Code correctness:

• The in-place assignment pattern is safe because CircularBuffer.__getitem__ returns indexed views
• The _all_initialized flag is correctly reset on buffer reset
• Initialization logic still handles first-push correctly before setting the flag

Confidence Score: 4/5

• This PR is safe to merge with minimal risk - well-targeted performance optimizations
• The changes are focused performance optimizations that eliminate GPU-CPU synchronization overhead without altering logic. The coordinated changes across three files (caching max_length, skipping checks after warmup, removing clone, using in-place assignment) work together correctly. Minor deduction because there are no new tests to verify the optimizations don't break edge cases, though the logic appears sound.
• No files require special attention

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/utils/buffers/circular_buffer.py	Optimized to avoid GPU-CPU synchronization in hot path by caching max_length as integer and tracking initialization state
source/isaaclab/isaaclab/utils/buffers/delay_buffer.py	Removed unnecessary .clone() call since consumer now uses in-place assignment
source/isaaclab/isaaclab/actuators/actuator_pd.py	Changed to in-place assignment to avoid unnecessary tensor allocation and copying

Sequence Diagram

sequenceDiagram
    participant Actuator as DelayedPDActuator
    participant DelayBuf as DelayBuffer
    participant CircBuf as CircularBuffer
    
    Note over Actuator,CircBuf: Hot Path (called every physics step)
    
    Actuator->>DelayBuf: compute(control_action.joint_positions)
    DelayBuf->>CircBuf: append(data)
    
    alt First time after reset
        CircBuf->>CircBuf: Check _all_initialized flag (false)
        CircBuf->>CircBuf: Check is_first_push = (_num_pushes == 0)
        CircBuf->>CircBuf: Call .any().item() (GPU sync)
        CircBuf->>CircBuf: Initialize buffer if needed
        CircBuf->>CircBuf: Set _all_initialized = true
    else All batches initialized (optimized path)
        CircBuf->>CircBuf: Skip initialization check
        Note over CircBuf: No GPU-CPU sync needed!
    end
    
    CircBuf->>CircBuf: Increment _num_pushes
    DelayBuf->>CircBuf: __getitem__(time_lags)
    CircBuf-->>DelayBuf: Return delayed data (view)
    DelayBuf-->>Actuator: Return delayed data (no clone)
    Actuator->>Actuator: In-place assign with [:]
    
    Note over Actuator,CircBuf: Optimizations Applied:<br/>1. Cached max_length as int (avoid .item())<br/>2. Skip initialization check after warmup<br/>3. Removed unnecessary .clone()<br/>4. In-place assignment in actuator

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