feat(paddlejs-backend-cpu): add flatten_continuous_range and pool2d_max

[liu2333hui]

FEATURE
加了新的CPU Ops, 包括flatten_continuous_range 和 pool2d_max.

DETAILS
Pool2d_max is based on the pre-existing pool2d implementation with some minor modifications.

Flatten_continuous_range is a deep clone of the input tensor to the output tensor.

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