[core][complex ndarray] Remove Complex DType

[shivasankarka]

Summary

Previously, the distinction between NDArray and ComplexNDArray was partially encoded through the use of DType and CDType. This PR simplifies the type system by removing the custom CDType datatype and replacing it with the standard DType. The use of CDType introduced issues with Mojo's compile-time type conversions, resulting in suboptimal user ergonomics and problems when converting between datatypes. To address this, we are standardizing on DType moving forward.

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New Syntax Proposal

To clearly differentiate between functions that create or manipulate standard and complex arrays, we introduce a simple naming convention: append a C suffix to functions returning complex arrays since Mojo does not support function overloading based on return type. The following is one such example,

fn ones[dtype: DType = DType.float64](shape: NDArrayShape) raises -> NDArray[dtype]
    pass

fn onesC[dtype: DType = DType.float64](shape: NDArrayShape) raises -> ComplexNDArray[dtype]
    pass

This approach keeps the API intuitive, avoids return-type ambiguity, and aligns better with Mojo's type system and simplifies a lot of type conversions.

[Copilot]

Copilot reviewed 4 out of 14 changed files in this pull request and generated no comments.

Files not reviewed (10)

• numojo/init.mojo: Language not supported
• numojo/core/init.mojo: Language not supported
• numojo/core/complex/init.mojo: Language not supported
• numojo/core/complex/complex_simd.mojo: Language not supported
• numojo/core/datatypes.mojo: Language not supported
• numojo/core/matrix.mojo: Language not supported
• numojo/prelude.mojo: Language not supported
• numojo/routines/io/formatting.mojo: Language not supported
• tests/core/test_complexArray.mojo: Language not supported
• tests/core/test_complexSIMD.mojo: Language not supported

[forfudan]

@shivasankarka It is absolutely a good move. ComplexDType is not very necessary since the real and imaginary parts can be decomposed into two DTypes, and the operations on complex arrays can be decomposed into operations on normal arrays.

Another nice thing is that we do not need to worry about the license anymore. Maybe we can remove the following paragraph from the readme file.

This project includes code from Mojo Standard Library, licensed under the Apache License v2.0 with LLVM Exceptions (see the LLVM License). MAX and Mojo usage and distribution are licensed under the MAX & Mojo Community License.