Add common VectorType alias

[sjfeng1999]

Motivation

Technical Details

Test Plan

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[Copilot]

Pull request overview

This PR introduces a set of common fixed-lane Vector alias types (e.g., Float32x4) in flydsl.expr.typing, along with a small utility descriptor to lazily compute per-class attributes (used to compute MLIR ir_type within an active ir.Context).

Changes:

• Added lazy_classattr descriptor in flydsl.expr.utils for non-cached, context-dependent class attributes.
• Added VectorAlias(...) factory and many concrete vector alias classes (e.g., Float32x4, Int8x16) plus __all__ exports.
• Added unit tests validating alias behavior and export via flydsl.expr.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/unit/test_vector.py	Adds unit tests for new vector alias types and package-level exports.
python/flydsl/expr/utils/__init__.py	Adds lazy_classattr descriptor used by alias types to compute ir_type lazily per access.
python/flydsl/expr/typing.py	Implements VectorAlias and many concrete *xN vector alias classes; updates vector behavior and exports.

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