Move SparseGPTModifier location with backwards compatibility

[kylesayrs]

Purpose

• Move SparseGPTModifier to a more logical folder to reduce confusion about "optimal brain compression / quantization (obcq)" being used to describe a sparsity-only algorithm but not being used to describe the gptq quantization algorithm

Changes

• Moved SparseGPTModifier from llmcompressor.modifiers.obcq to llmcompressor.modifiers.pruning.sparsegpt
• Left a depreciation warning and backwards compatible import

warnings.warn(
    "llmcompressor.modifiers.obcq has been moved to "
    "llmcompressor.modifiers.pruning.sparsegpt Please update your paths",
    DeprecationWarning,
)

Testing

• grepped codebase grep -r 'import SparseGPTModifier' src tests examples
• on-commit tests

[github-actions]

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