Remove CMake references in CI and README (#241)

Author: vmoens

.circleci/unittest/linux/scripts/environment.yml

@@ -3,7 +3,6 @@ channels:
   - defaults
 dependencies:
   - pip
-  - cmake >= 3.18
   - protobuf
   - pip:
     - hypothesis

.circleci/unittest/linux_optdeps/scripts/environment.yml

@@ -3,7 +3,6 @@ channels:
   - defaults
 dependencies:
   - pip
-  - cmake >= 3.18
   - pip:
     - hypothesis
     - future

.circleci/unittest/linux_stable/scripts/environment.yml

@@ -4,7 +4,6 @@ channels:
 dependencies:
   - pip
   - ninja
-  - cmake >= 3.18
   - protobuf
   - pip:
     - hypothesis

README.md

@@ -44,12 +44,11 @@ and many more to come!
 
 ## Installation
 Create a conda environment where the packages will be installed. 
-Before installing anything, make sure you have the latest version of `cmake` and `ninja` libraries:
+Before installing anything, make sure you have the latest version of the `ninja` library:
 
 ```
 conda create --name torch_rl python=3.9
 conda activate torch_rl
-conda install cmake -c conda-forge
 pip install ninja
 ```
 
@@ -134,8 +133,6 @@ python -c 'from torchrl.envs import GymEnv'
 ```
 If this is the case, consider executing torchrl from another location.
 
-This may also be caused by several dependency issues: cmake, gcc or ninja versioning, or absence of the CuDNN library when working in a CUDA environment. 
-
 On **MacOs**, we recommend installing XCode first. 
 With Apple Silicon M1 chips, make sure you are using the arm64-built python (e.g. [here](https://betterprogramming.pub/how-to-install-pytorch-on-apple-m1-series-512b3ad9bc6)). Running the following lines of code