Add FLOPs analysis tool for groundingdino

## Motivation

The existing [tools/analysis_tools/get_flops.py](cci:7://file:///home/david/mmdetection/tools/analysis_tools/get_flops.py:0:0-0:0) does not support grounding / vision-language detection models (e.g., GroundingDINO, GroundingCLIP) because these models require text inputs and have multi-modal architectures that cannot be traced end-to-end with `mmengine.analysis.get_model_complexity_info`.

This PR adds a dedicated FLOPs analysis tool that handles the unique architecture of grounding detection models, providing per-component FLOPs and parameter breakdowns.

## Modification

**New file: [tools/analysis_tools/get_flops_grounding.py](cci:7://file:///home/david/mmdetection/tools/analysis_tools/get_flops_grounding.py:0:0-0:0)**

A script that computes per-component FLOPs and parameter counts for grounding detection models:
- **Vision Backbone**: Accurate FLOPs via `fvcore.nn.FlopCountAnalysis`
- **Text Encoder**: Estimated FLOPs based on model type (CLIP, BERT, etc.)
- **Neck (ChannelMapper)**: Estimated from config-driven channel/stride info
- **Transformer Encoder/Decoder**: Estimated from config-driven architecture params
- **Detection Head**: Parameter count

Key design choices:
- Automatically disables `with_cp` (gradient checkpointing) which is incompatible with JIT tracing, without modifying the original config
- Reads architecture parameters (channels, layers, embed_dim, etc.) dynamically from the model config instead of hardcoding
- Uses `MMLogger` consistent with existing mmdet tools

**New file: [tests/test_tools/test_get_flops_grounding.py](cci:7://file:///home/david/mmdetection/tests/test_tools/test_get_flops_grounding.py:0:0-0:0)**

41 unit tests covering all helper functions and config readers.

## BC-breaking

No. This PR only adds new files and does not modify any existing code.

## Use cases

```bash
# Basic usage
python tools/analysis_tools/get_flops_grounding.py \
    configs/mm_grounding_dino/grounding_dino_swin-t_finetune_8xb4_20e_cat.py

# Custom input shape
python tools/analysis_tools/get_flops_grounding.py <config> --shape 640 640

# Specify number of classes for text encoder FLOPs estimation
python tools/analysis_tools/get_flops_grounding.py <config> --num-classes 80

Author: 0xyangl