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Support using a stage in memory, rather the default stage attached to a
usd context, for faster operations during stage initialization and
cloning.

The feature requires a change on Isaac Sim which is tracked separately.

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- New feature (non-breaking change which adds functionality)

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- [ ] My changes generate no new warnings
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---------
Signed-off-by: Kelly Guo <kellyguo123@hotmail.com>
Signed-off-by: rwiltz <165190220+rwiltz@users.noreply.github.com>
Signed-off-by: Ashwin Varghese Kuruttukulam <123109010+ashwinvkNV@users.noreply.github.com>
Signed-off-by: Kelly Guo <kellyg@nvidia.com>
Signed-off-by: Michael Gussert <michael@gussert.com>
Co-authored-by: peterd-NV <peterd@nvidia.com>
Co-authored-by: Kelly Guo <kellyg@nvidia.com>
Co-authored-by: cosmith-nvidia <141183495+cosmith-nvidia@users.noreply.github.com>
Co-authored-by: jaczhangnv <jaczhang@nvidia.com>
Co-authored-by: rwiltz <165190220+rwiltz@users.noreply.github.com>
Co-authored-by: Yanzi Zhu <yanziz@nvidia.com>
Co-authored-by: nv-mhaselton <mhaselton@nvidia.com>
Co-authored-by: CY Chen <cyc@nvidia.com>
Co-authored-by: oahmednv <oahmed@Nvidia.com>
Co-authored-by: Ashwin Varghese Kuruttukulam <123109010+ashwinvkNV@users.noreply.github.com>
Co-authored-by: Rafael Wiltz <rwiltz@nvidia.com>
Co-authored-by: Michael Gussert <michael@gussert.com>
Co-authored-by: Kelly Guo <kellyguo123@hotmail.com>
Co-authored-by: chengronglai <chengrongl@nvidia.com>
Co-authored-by: pulkitg01 <pulkitg@nvidia.com>
Co-authored-by: Connor Smith <cosmith@nvidia.com>
Co-authored-by: Ashwin Varghese Kuruttukulam <ashwinvk@nvidia.com>
Co-authored-by: lotusl-code <lotusl@nvidia.com>