Pre-training Objectives for LLMs
✓ Pre-training is the foundational stage in developing Large Language Models (LLMs).
✓ It involves exposing the model to massive text datasets and training it to learn grammar, structure, meaning, and reasoning before it is fine-tuned for specific tasks.
✓ The objective functions used during pre-training determine how effectively the model learns language representations.
→ Why Pre-training Matters
✓ Teaches the model general linguistic and world knowledge.
✓ Builds a base understanding of syntax, semantics, and logic.
✓ Reduces data requirements during later fine-tuning.
✓ Enables the model to generalize across multiple domains and tasks.
→ Main Pre-training Objectives
1. Causal Language Modeling (CLM)
✓ Also known as Autoregressive Training, used by models like GPT.
✓ Objective → Predict the next token given all previous tokens.
✓ Example:
→ Input: “The sky is” → Target: “blue”
✓ The model learns word sequences and context flow — ideal for text generation and completion.
✓ Formula (simplified):
→ Maximize P(w₁, w₂, ..., wₙ) = Π P(wᵢ | w₁, ..., wᵢ₋₁)
2. Masked Language Modeling (MLM)
✓ Introduced with BERT, a bidirectional training objective.
✓ Objective → Predict missing words randomly masked in a sentence.
✓ Example:
→ Input: “The [MASK] is blue.” → Target: “sky”
✓ Allows the model to see context from both left and right, capturing deeper semantic relationships.
✓ Formula (simplified):
→ Maximize P(masked_token | visible_tokens)
3. Denoising Autoencoding
✓ Used by models like BART and T5.
✓ Objective → Corrupt the input text (e.g., mask, shuffle, or remove parts) and train the model to reconstruct the original sentence.
✓ Encourages robust understanding and recovery of meaning from noisy or incomplete inputs.
✓ Example:
→ Input: “The cat ___ on the mat.” → Target: “The cat sat on the mat.”
4. Next Sentence Prediction (NSP)
✓ Used alongside MLM in early BERT training.
✓ Objective → Predict whether one sentence logically follows another.
✓ Example:
→ Sentence A: “He opened the door.”
→ Sentence B: “He entered the room.” → Label: True
✓ Helps the model learn coherence and discourse-level relationships.
5. Permutation Language Modeling (PLM)
✓ Used by XLNet, combining autoregressive and bidirectional learning.
✓ Objective → Predict tokens in random order rather than fixed left-to-right.
✓ Enables the model to capture broader context and dependencies without masking.
6. Contrastive Learning Objectives
✓ Used in multimodal and instruction-based pretraining.
✓ Objective → Maximize similarity between semantically related pairs (e.g., a caption and its image) and minimize similarity between unrelated pairs.
✓ Builds robust cross-modal and conceptual understanding.
→ Modern Combined Objectives
✓ Modern LLMs often merge multiple pre-training objectives for richer learning.
✓ Example:
→ T5 uses denoising + text-to-text generation.
→ GPT-4 expands causal modeling with instruction-tuned objectives and reinforcement learning (RLHF).
✓ These hybrid objectives enable models to perform a wide range of generative and comprehension tasks effectively.
→ Quick tip
✓ Pre-training objectives teach LLMs how to predict, reconstruct, and reason over text.
✓ CLM → next-word prediction.
✓ MLM → masked token recovery.
✓ Denoising & NSP → structure and coherence.
✓ Contrastive → cross-domain learning.
✓ Together, they form the foundation for the deep understanding and fluency that define modern LLMs.
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