nixlbench: change to range lookup in Neuron allocation tracking (#1833)

Today in nixlbench, when data is copied from Neuron device to host, it
checks the address of an IOV within the existing Neuron memory
allocation. The latter is done once with a single large buffer, which
has the base address different from the IOV's base address. Not found
neuron allocation leads to nixlbench failure.

## What?
Change to range lookup when searching in Neuron allocation tracking.




<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Fixed device copy and memset to operate on the correct region within
an existing allocation, using the full VA range (not just the base).
* Improved device allocation tracking to reliably resolve tensors for
any VA within a mapped range.
* Added stronger bounds checking for copy and fill operations to prevent
out-of-range reads and writes.
* Enhanced diagnostics with clearer messages when allocations are
missing, access is out of bounds, or device read/write calls fail.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: Adit Ranadive <aranadive@nvidia.com>