---
id: wiki-2026-0508-encoder-decoder-inconsistency
title: Encoder Decoder Inconsistency
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [encoder-decoder-mismatch, seq2seq-inconsistency]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [nlp, transformer, training, decoding]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: pytorch
---

# Encoder Decoder Inconsistency

## 매 한 줄
> **"매 encoder가 본 분포 ≠ decoder가 생성하는 분포"**. Seq2seq training 시 encoder는 ground-truth context를 보지만 decoder는 inference에서 자기 prediction을 다시 입력으로 받기 때문에 train/inference 간 distribution shift가 발생한다. 매 exposure bias 의 근본 원인.

## 매 핵심

### 매 정의
- **Train**: decoder input = teacher-forced ground truth.
- **Inference**: decoder input = previously generated token.
- **Gap**: 매 error compounds along sequence — early mistake → later tokens conditioned on out-of-distribution prefix.

### 매 표현
- Exposure bias (Ranzato 2016).
- Schedule sampling 의 motivation.
- Hallucination 의 한 원인 (특히 long-form generation).

### 매 응용
1. NMT (Neural Machine Translation) — 매 long sentence translation degradation.
2. Summarization — repetition / drift.
3. Speech recognition — RNN-T vs CTC trade-off.
4. Code generation — 매 long completion 의 syntax break.

## 💻 패턴

### Scheduled Sampling
```python
import torch
import torch.nn.functional as F

def scheduled_sampling_step(decoder, prev_token, hidden, gt_token, p_use_gt: float):
    """p_use_gt 의 확률로 ground-truth, 아니면 model prediction 의 사용."""
    if torch.rand(1).item() < p_use_gt:
        input_tok = gt_token
    else:
        with torch.no_grad():
            logits, _ = decoder(prev_token, hidden)
            input_tok = logits.argmax(dim=-1)
    out_logits, hidden = decoder(input_tok, hidden)
    return out_logits, hidden
```

### Minimum Risk Training
```python
def mrt_loss(model, src, refs, n_samples=8):
    """매 sequence-level loss 의 — 매 sampled hypotheses 에 대해 risk minimize."""
    hyps = [model.sample(src) for _ in range(n_samples)]
    risks = torch.tensor([1 - bleu(h, refs) for h in hyps])
    log_probs = torch.stack([model.log_prob(h, src) for h in hyps])
    weights = F.softmax(log_probs, dim=0)
    return (weights * risks).sum()
```

### Self-distillation Fix
```python
def self_distill(student, teacher, src, T=2.0):
    """매 teacher 가 자기 생성한 sequence 의 사용 — 매 train/inference gap 축소."""
    with torch.no_grad():
        gen = teacher.generate(src, do_sample=True, top_p=0.9)
        teacher_logits = teacher(src, gen).logits
    student_logits = student(src, gen).logits
    return F.kl_div(
        F.log_softmax(student_logits / T, dim=-1),
        F.softmax(teacher_logits / T, dim=-1),
        reduction="batchmean",
    ) * T * T
```

### Beam Search with Length Penalty
```python
def length_penalty(score, length, alpha=0.7):
    """GNMT length penalty — 매 short hypothesis 의 bias 보정."""
    return score / ((5 + length) ** alpha / (5 + 1) ** alpha)
```

### Contrastive Decoding
```python
def contrastive_decode(big, small, prompt, alpha=0.5):
    """매 large model logit − small model logit — 매 expert/amateur gap 의 강조."""
    big_logits = big(prompt).logits[:, -1]
    small_logits = small(prompt).logits[:, -1]
    return big_logits - alpha * small_logits
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Short sequence (<32) | Teacher forcing 충분 |
| Long sequence | Scheduled sampling / MRT |
| Production NMT | Beam + length penalty + coverage |
| LLM long-form | Contrastive decoding / self-distillation |

**기본값**: teacher forcing + 1k step warmup 이후 scheduled sampling.

## 🔗 Graph
- 부모: [[Sequence-to-Sequence]] · [[Transformer]]
- Adjacent: [[Hallucination]]

## 🤖 LLM 활용
**언제**: long-form generation 의 quality issue 분석 시. Train/eval BLEU gap 의 진단.
**언제 X**: 매 short classification — 매 inconsistency 의 무관.

## ❌ 안티패턴
- **Pure teacher forcing forever**: 매 inference distribution 의 미본 채 deploy.
- **Greedy decoding only**: 매 early mistake 의 lock-in.
- **No length normalization**: beam 의 short hypothesis bias.

## 🧪 검증 / 중복
- Verified (Ranzato et al. 2016, Bengio et al. 2015 scheduled sampling).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — encoder/decoder distribution shift + scheduled sampling/MRT/contrastive decoding |
