Project does not compile without vllm feature flag

[eureka-cpu]

If this is a default feature it should be enabled by default, otherwise if it's an optional feature it shouldn't be causing a panic when attempting to compile without it.

[eureka-cpu]

For instance, when trying to run some tests that are unrelated to vllm:

eureka@pop-os:~/Code/Atoma/atoma-node-inference$ cargo test json_schema_tests
+ command cargo test json_schema_tests
  Downloaded fdeflate v0.3.5
  Downloaded 1 crate (26.8 KB) in 0.33s
   Compiling log v0.4.22
   Compiling tracing-core v0.1.32
   Compiling indexmap v2.5.0
   Compiling hashbrown v0.14.5
warning: [email protected]: Build directory: "/home/eureka/Code/Atoma/atoma-node-inference/target/debug/build/csrc-fe7d8c1e894e2bf2/out"
   Compiling fdeflate v0.3.5
   Compiling raw-cpuid v11.1.0
   Compiling metrics v0.22.3
   Compiling overload v0.1.1
   Compiling sketches-ddsketch v0.2.2
   Compiling sharded-slab v0.1.7
   Compiling thread_local v1.1.8
   Compiling metrics v0.23.0
   Compiling atoma-backends v0.1.0 (/home/eureka/Code/Atoma/atoma-node-inference/backends)
   Compiling nu-ansi-term v0.46.0
error: vllm backend is not enabled. Please compile with --features vllm
 --> backends/src/lib.rs:5:1
  |
5 | compile_error!("vllm backend is not enabled. Please compile with --features vllm");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `atoma-backends` (lib test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...