Convert base_linear_op to a dense linear operator in BlockDiagLinearOperator

[kayween]

This PR would fix #118.

In general, many operations in BlockDiagLinearOperator assume that base_linear_op is a linear operator. Thus, it's better to convert it into a linear operator in the constructor.

Added test cases to cover the change.

[Copilot]

Pull request overview

This PR fixes issue #118 by ensuring that tensor inputs to BlockDiagLinearOperator are properly converted to DenseLinearOperator instances before being processed. This is necessary because the parent class BlockLinearOperator calls LinearOperator-specific methods like _permute_batch() on base_linear_op when block_dim != -3, which would fail for raw tensors.

Key Changes:

• Added tensor-to-LinearOperator conversion in BlockDiagLinearOperator.__init__()
• Added test class TestBlockDiagLinearOperatorFromTensor to validate tensor initialization
• Minor style improvement removing unnecessary parentheses in tuple unpacking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
linear_operator/operators/block_diag_linear_operator.py	Added import of DenseLinearOperator and conversion logic in constructor to wrap tensor inputs before passing to parent class
test/operators/test_block_diag_linear_operator.py	Added _initialize_from_tensor flag and new test class to verify tensor initialization works correctly, plus minor style fix

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[Balandat]

thanks, lgtm