refactor(lazy import): centralize, optimize, CPU fallback when broken…

[lmeyerov]

Fixes colab until #517 gets figured out

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Fixed

• Graceful CPU fallbacks: When lazy GPU dependency imports throw ImportError, now warns and falls back, instead of crashing. Commonly seen due to:

  • GPU libs installed but broken CUDA environments
  • GPU libs installed but no GPU

• Ring layouts handle filtered DF inputs, no longer giving unexpected positions

• g.encode_axis(...) updates now functional; were erroneously doing global inplace updates

Changed

• Centralize lazy imports into graphistry.utils.lazy_import
• Lazy imports distinguish ModuleNotFound (=> False) from ImportError (warn + False)

Optimized

More cudf sniffing without an import