Document on unsupported tensor types.

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See https://pkg.go.dev/gorgonia.org/tensor#pkg-variables

and https://onnx.ai/onnx/intro/concepts.html#supported-types

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Operators with unsupported types:

• Abs: bfloat16, float16
• Acos: float16
• Acosh: float16
• Add: bfloat16, float16
• Asin: float16
• Asinh: float16
• Tanh: float16
• Atanh: float16
• Cast: bfloat16, float16, string
• Constant: bfloat16, float16, string, complex64, complex128
• Conv: float16
• Cos: float16
• Cosh: float16
• Div: float16
• Equal: bfloat16, float16, string
• Expand: float16
• Flatten: bfloat16, float16, string, complex64, complex128
• Gather: bfloat16, float16, string, complex64, complex128
• Gemm: float16

Stopped because all others are basically same