Fix Inefficient default GELU implementation in GPT2(#39073)
Author: wenboqian
Pull request: yes

# What does this PR do?

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Fixes https://github.com/huggingface/transformers/issues/39073 (https://github.com/huggingface/transformers/issues/39073)

### Environment
- CUDA 12.8
- Ubuntu 20.04
- torch 2.8.0
- transformers 4.55.0

## Fix
Replace GPT-2 configuration default `activation_function` from `"gelu_new"` (uses NewGELUActivation) to `"gelu"` (uses GELUActivation).

Location: `src/transformers/models/gpt2/configuration_gpt2.py`, line 146
```python
# Before
activation_function="gelu_new",
# After
activation_function="gelu",
```
### Performance Improvement
### Inference per iteration (Before vs After)
- throughput: +12.0% (1.12x)
- latency: -10.6% (1.12x faster)

- Activation
  - Before: gelu_new
  - After: gelu

### Benchmark summary (comparison)
| Metric | Before | After | Change |
| --- | ---: | ---: | ---: |
| Mean time (ms) | 1.307 | 1.168 | -10.6% (1.12x faster) |
| Throughput (tokens/s) | 6,265,653.57 | 7,014,827.81 | +12.0% (1.12x) |
| Tokens/iter | 8,192 | 8,192 | — |

## Testing
### Configuration
- Model: sshleifer/tiny-gpt2
- batch_size: 16
- seq_len: 512
- iterations: 100
#### config.json:
```json
{
  "architectures": [
    "GPT2LMHeadModel"
  ],
  "attn_pdrop": 0.1,
  "bos_token_id": 50256,
  "embd_pdrop": 0.1,
  "eos_token_id": 50256,
  "initializer_range": 0.02,
  "layer_norm_epsilon": 1e-05,
  "model_type": "gpt2",
  "n_ctx": 1024,
  "n_embd": 2,
  "n_head": 2,
  "n_layer": 2,
  "n_positions": 1024,
  "resid_pdrop": 0.1,
  "summary_activation": null,
  "summary_first_dropout": 0.1,
  "summary_proj_to_labels": true,
  "summary_type": "cls_index",
  "summary_use_proj": true,
  "task_specific_params": {
    "text-generation": {
      "do_sample": true,
      "max_length": 50
    }
  },
  "vocab_size": 50257
}
```

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gpt2