Add reshape function.

[hzhangxyz]

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[codecov]

Codecov Report

❌ Patch coverage is 3.50877% with 110 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
grassmann_tensor/tensor.py	3.50%	110 Missing ⚠️

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[Copilot]

Pull Request Overview

This PR adds a reshape function to the GrassmannTensor class that allows reshaping of Grassmann tensors while handling the complex requirements of splitting and merging edges with proper parity sign calculations.

• Adds a new _reorder_indices helper method for calculating edge reordering and parity signs
• Implements the main reshape method that can split or merge tensor dimensions while preserving Grassmann algebra properties
• Handles complex logic for maintaining arrow consistency and applying appropriate signs during tensor transformations

[Copilot]

The loop termination condition is placed after potential IndexError scenarios. If either cursor exceeds bounds during the loop iterations, the function will crash before reaching this check.

Suggested change

if cursor_plan == len(new_shape) and cursor_self == self.tensor.dim():

[Copilot]

The condition self.arrow[index] uses the original tensor's arrow, but index refers to positions in the original tensor during splitting. After tensor reshaping, the dimensions may not correspond correctly.

Suggested change

	(self._unsqueeze(sign, index, self.tensor.dim()) for index, sign in splitting_sign if self.arrow[index]),
	(self._unsqueeze(sign, index, self.tensor.dim()) for index, sign in splitting_sign if arrow[index]),